Heat olive oil in a skillet over medium heat.
Add diced onion and bell pepper; sauté 2–3 minutes until softened.
Stir in cooked chicken and sprinkle chili powder, cumin, smoked paprika, garlic powder, salt and pepper.
Cook chicken 3–4 minutes until heated through and spices are fragrant; remove from heat.
Wipe skillet lightly and return to medium-low heat for grilling tortillas.
Place one tortilla in skillet and evenly sprinkle about 1/3–1/4 cup shredded cheese over half the tortilla.
Spoon a quarter of the chicken mixture over the cheese and add cilantro if using.
Fold tortilla in half and press gently with a spatula to flatten and seal.
Cook 2–3 minutes per side until golden brown and cheese is fully melted, flipping once.
Transfer to a cutting board, let rest 1 minute, then slice into wedges and serve with salsa, guacamole or sour cream.