In a bowl, toss the cooked chicken with chili powder, cumin, salt, and pepper. If your chicken is already well-seasoned, you can skip this step or just warm it in a skillet before assembling.
Lay one tortilla flat and sprinkle half the shredded cheese over one half of the tortilla. Layer on the seasoned chicken, bell pepper, onion (if using), and the remaining cheese. Fold the tortilla in half to create a half-moon shape.
Heat olive oil or butter in a skillet over medium heat. Place the folded quesadilla in the skillet and cook for 2–3 minutes per side, or until the tortilla is golden and crispy and the cheese is fully melted.
Remove from heat and let cool for 1 minute. Slice into wedges and serve with your favorite dips like salsa, sour cream, or guacamole.