If using raw chicken, season it with chili powder, paprika, garlic powder, cumin, salt, and black pepper. Cook in a skillet with oil until fully cooked and the internal temperature reaches 165°F (74°C). Shred or dice the chicken.
If using bell pepper and onion, cook them in the skillet for 3–5 minutes until slightly softened.
Return the cooked chicken to the skillet and stir everything together. Remove from the heat.
Sprinkle a thin layer of cheddar and Monterey Jack cheese over half of each tortilla.
Divide the chicken mixture between the tortillas and top with the remaining cheese. Fold each tortilla in half.
Heat a skillet over medium heat and lightly grease it with oil.
Cook each quesadilla for 2–4 minutes per side, until golden brown and crispy and the cheese has melted.
Transfer the quesadillas to a cutting board and let them rest for 1–2 minutes.
Cut into wedges, garnish with cilantro if desired, and serve warm with salsa, sour cream, or guacamole.