Whisk lime juice, olive oil, smoked paprika, cumin, salt, garlic, jalapeño, and oregano. Add chicken thighs and coat thoroughly, turning the inner side of each thigh outward so the marinade penetrates the meat. Marinate 30–60 minutes in the fridge.
Mash avocados with sour cream, lime juice, garlic, salt, cilantro (if using), and pepper until smooth but spoonable. Taste and adjust salt and acidity. Chill.
Warm beans with olive oil, cumin, paprika, salt, and pepper over low heat. Add a splash of stock or water to make them slightly saucy so they adhere to the tortilla. Remove from heat.
Sear chicken in a hot skillet (cast iron or heavy pan) with a thin film of oil: about 2–3 minutes per side to brown. Transfer pan to a 350°F / 175°C oven and cook 10–15 minutes, or until the internal temperature reaches 165°F / 74°C. Rest 5 minutes, then slice into strips.
Warm tortillas briefly so they’re pliable. For each tortilla, spread 3–4 tbsp beans in a strip, add ~100 g (3.5 oz) sliced chicken, 2 tbsp avocado cream, and ¼ cup (25 g) cheese. Fold in the sides, then roll up tightly like a burrito, keeping the seam underneath.
Heat oil to 350°F / 175°C in a wide skillet (shallow fry). Place chimichangas seam-side down and fry 2–3 minutes per side until evenly golden. Drain on a rack or paper towel.
Serve hot with extra avocado cream, lime wedges, and fresh herbs. Pair with an easy salad or rice (see links above).