Preheat the oven to 425°F. Line a large sheet pan with parchment paper.
In a large bowl, mix olive oil, Dijon mustard, lemon juice, garlic, paprika, oregano, onion powder, salt, and black pepper.
Add the chicken and coat well. Let rest for 15 minutes if time allows.
Add the potatoes, carrots, bell pepper, zucchini, and red onion. Toss with the remaining seasoning. Add a little more olive oil if needed.
Spread the vegetables on the sheet pan in one layer. Place the chicken pieces between or on top of the vegetables, keeping the chicken exposed.
Bake for 35 to 45 minutes for bone-in chicken, or 25 to 30 minutes for boneless thighs, until the chicken reaches 165°F and the vegetables are tender.
Broil for 2 to 3 minutes at the end for extra color if desired.
Finish with fresh lemon juice and chopped parsley. Serve hot.