There are days when you don’t want a complicated dinner, but you still want something hot, generous and packed with flavour. That’s where this bavette steak sandwich comes in: juicy flank steak, caramelized red onions, crunchy iceberg lettuce and a tangy harissa-mustard mayo inside a crisp baguette.

This recipe is based on my steak sandwich YouTube video, but I’ve adjusted the proportions, clarified the cooking times and refined the sauce so it’s even more reliable at home. If you love bistro-style steak, loaded sandwiches and easy weeknight dinners, this flank steak baguette deserves a spot in your rotation.

Why you’ll love this bavette steak sandwich

Perfect for steak lovers: Bavette (flank steak) is full of beefy flavour and stays juicy when sliced against the grain.
Fast enough for a weeknight: Ready in about 30 minutes, especially if you prep the onions ahead.
Harissa mayo with a kick: Mustard, mayo and harissa make a spicy, tangy sandwich sauce that clings to the meat and bread.
Inspired by my video, optimized for the blog: Same spirit as the YouTube recipe, but with updated sauce, better quantities and clearer instructions.
Great with classic sides: Serve with authentic Belgian fries, a simple homemade Greek salad or creamy mashed potatoes for a full bistro-style meal.

Ingredients

For the steak

300 g / 10.5 oz bavette (flank steak)
10 ml / 2 tsp vegetable oil, for searing
15 g / 1 tbsp butter
Salt and freshly ground black pepper

You can also use skirt steak or flap steak if you can’t find bavette.

For the caramelized onions

150 g / 5 oz red onion (about 1 medium), thinly sliced
10 ml / 2 tsp vegetable oil
10 g / 2 tsp butter
Small pinch of salt

For the harissa-mustard mayo

30 g / 2 tbsp Dijon mustard
20 g / 1 ½ tbsp mayonnaise (store-bought or homemade mayo)
5–10 g / 1–2 tsp harissa paste (adjust to taste)
5 ml / 1 tsp lemon juice or white wine vinegar
10 ml / 2 tsp olive oil or neutral oil
Salt and freshly ground black pepper, to taste

For assembly

1 medium baguette (about 250–300 g / 9–10.5 oz)
40–50 g / 1 ½–2 cups finely shredded iceberg lettuce
Optional: a few cherry tomatoes for serving

Makes 2 generous sandwiches or 1 very large baguette to share.

Step-by-step instructions

1. Caramelize the onions

Heat the vegetable oil in a skillet over medium heat.
Add the sliced red onion and a small pinch of salt. Stir to coat.
Cook gently for 12–15 minutes, stirring often, until the onions become soft and start to brown.
Add the butter and keep cooking for another 3–5 minutes until the onions are deeply golden and sweet, but not burned.
Remove from the pan and set aside.

If you love caramelized onions, you can make a larger batch ahead and keep them in the fridge for patty melts, burgers or even over homemade mashed potatoes.

2. Make the harissa-mustard mayo

In a small bowl, whisk together the mustard, mayonnaise, harissa, lemon juice (or vinegar) and oil.
Taste and season with salt and pepper.
Adjust the heat by adding a bit more harissa if you like it spicy.

You’re looking for a thick, creamy sauce that spreads easily and doesn’t run off the bread. If it’s too thick, add a few drops of water or lemon juice. If it’s too thin, whisk in a little more mayo.

This harissa mayo is also excellent in a patty melt sandwich or drizzled over chicken tenders.

3. Prep the baguette and lettuce

Slice the baguette lengthwise, but not all the way through if you prefer a “hinged” sandwich.
Lightly toast the cut sides of the bread in a dry pan or under the broiler for 1–2 minutes. This keeps it crisp once the steak juices hit.
Finely shred the iceberg lettuce and pat it dry if it’s wet. The lettuce will act as a barrier so the juice from the steak doesn’t soak the bread too quickly.

4. Cook the bavette (flank steak)

Pat the steak dry with paper towels.
Season generously on both sides with salt and freshly ground black pepper.
Heat a heavy skillet (cast iron is ideal) over medium-high to high heat until very hot.
Add the vegetable oil. When it shimmers, carefully lay the steak in the pan.
Sear for about 30 seconds, then flip. Continue flipping every 30–40 seconds to build a nice crust without overcooking.
After 1–2 minutes, add the butter and baste the steak with the foaming butter.
Cook until the steak reaches about 52–54°C / 125–130°F for medium-rare. This will usually take 4–6 minutes depending on thickness.
Transfer the steak to a cutting board and let it rest 5–7 minutes.

Resting is essential: it keeps the meat juicy and prevents all the juices from soaking into the bread.

5. Slice and assemble the sandwich

Slice the rested steak thinly against the grain.
Spread a generous layer of harissa-mustard mayo on both sides of the toasted baguette.
Add a layer of shredded iceberg lettuce on the bottom.
Pile on the sliced bavette.
Top with a layer of caramelized onions.
Drizzle with a little extra harissa mayo if you like.
Add a bit more lettuce for crunch, then close the sandwich.
Slice into 2 portions and serve right away, with cherry tomatoes or pickles on the side.

Tips and tricks for the best steak sandwich

Choose the right cut: Bavette (flank steak) is ideal, but skirt steak or flap steak also work beautifully. For a more classic experience, you can serve this with steakhouse-style pepper sauce or Roquefort sauce for steak.
Flip often: Flipping every 30–40 seconds gives you a great crust and even cooking, just like in the video.
Don’t skip the rest: This is the difference between a juicy sandwich and a soggy, dry one.
Use good bread: A crisp baguette is classic, but you can also use ciabatta or a rustic roll.
Balance the sauce: If the harissa is very strong, start with less and build up. If the sauce is too spicy, add more mayo to calm it down.

Substitutions and variations

No harissa? Replace it with sriracha, another chili paste or your favourite hot sauce. For a smoky note, use chipotle in adobo.
Different cut of beef: Flat iron, hanger steak or even thin-sliced ribeye work well. Adjust cooking time for thickness.
Add cheese: Turn this into a “steak and cheese” baguette by adding slices of provolone, mozzarella or aged cheddar and briefly melting it under the broiler.
Sauce variations: Swap the harissa mayo for homemade chimichurri or drizzle with Roquefort steak sauce for a different twist.
Make it a full sandwich night: Serve this alongside juicy smash burgers with bacon and cheddar or my ultimate flank steak sandwich with chimichurri.

What to serve with this bavette steak sandwich

This sandwich is rich and flavourful, so simple sides are perfect:

Authentic Belgian fries
Homemade ketchup and garlic croutons on a side salad
Homemade Greek salad or Greek pasta salad with creamy dressing
A bowl of creamy tomato soup or creamy pumpkin soup
For dessert, try moist dark chocolate cake or no-churn dark chocolate ice cream.

FAQ

Can I make this steak sandwich ahead of time?

The steak itself can be cooked, cooled and sliced ahead, then gently rewarmed in a pan with a splash of stock or its own juices. For the best texture, assemble the sandwich just before serving so the bread stays crisp and the lettuce remains crunchy.

Can I use another type of bread?

Yes. Ciabatta, sourdough rolls, hoagie rolls or even toasted sandwich bread work. For a more “American diner” twist, try the filling on toasted sandwich bread like a patty melt and serve it with crispy fried calamari or crispy beef.

How spicy is the harissa mayo?

With 1 teaspoon of harissa it’s gently spicy. At 2 teaspoons it has a noticeable kick, similar to a medium hot sauce. Start low and adjust to your tolerance.

Can I make this without mayonnaise?

Yes. Replace the mayo with thick Greek yogurt or sour cream. You can also try a mix of yogurt and a little olive oil for a lighter, tangy sauce.

Can I grill the steak instead of pan-searing?

Absolutely. Grill the bavette over high heat, flipping once or twice, until it reaches medium-rare. Let it rest and slice as directed. A grilled version is excellent served with homemade Caesar salad and Parisian potatoes.

Ultimate Bavette Steak Sandwich with Harissa Mayo (Flank Steak Baguette)

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Prep Time 15

Cooking Time 15

Category Appetizer, Main Course

Cuisine American, French

Portions 2 Portions

Calories 500 kcal

Ingredients

For the steak

300 g / 10.5 oz bavette flank steak
10 ml / 2 tsp vegetable oil
15 g / 1 tbsp butter
Salt and freshly ground black pepper

For the caramelized onions

150 g / 5 oz red onion 1 medium, thinly sliced
10 ml / 2 tsp vegetable oil
10 g / 2 tsp butter
Small pinch of salt

For the harissa-mustard mayo

30 g / 2 tbsp Dijon mustard
20 g / 1 ½ tbsp mayonnaise
5 –10 g / 1–2 tsp harissa paste
5 ml / 1 tsp lemon juice or white wine vinegar
10 ml / 2 tsp olive oil or neutral oil
Salt and freshly ground black pepper to taste

For assembly

1 medium baguette 250–300 g / 9–10.5 oz
40 –50 g / 1 ½–2 cups finely shredded iceberg lettuce
Cherry tomatoes or pickles for serving (optional)

Instructions

Caramelize the onions: Heat oil in a skillet over medium heat. Add sliced red onion and a pinch of salt. Cook for 12–15 minutes, stirring often, until soft and lightly browned. Add butter and cook 3–5 minutes more until deep golden. Remove from the pan and set aside.
Make the harissa mayo: In a small bowl, whisk together mustard, mayonnaise, harissa, lemon juice and oil. Season with salt and pepper and adjust the heat level to taste.
Prep the bread and lettuce: Slice the baguette lengthwise and lightly toast the cut sides in a dry pan or under the broiler for 1–2 minutes. Shred the iceberg lettuce.
Cook the steak: Pat the bavette dry and season generously with salt and pepper. Heat a heavy skillet over high heat, add the oil, then sear the steak, flipping every 30–40 seconds, until a crust forms and it reaches 52–54°C / 125–130°F (medium-rare), about 4–6 minutes. Add butter during the last minutes and baste. Transfer to a board and rest 5–7 minutes.
Slice and assemble: Slice the rested steak thinly against the grain. Spread harissa mayo on both sides of the toasted baguette. Add a layer of shredded lettuce, then the sliced steak, then the caramelized onions. Drizzle with a bit more sauce and add extra lettuce if desired. Close the baguette and slice into 2 portions.
Serve: Serve immediately with your favourite sides, such as fries, salad or soup.

Video

Notes