Preheat the oven to 425°F (220°C) and grease a large sheet pan with cooking spray or melted butter.
Lay three tortillas across the bottom of the pan, slightly overlapping them and letting the edges hang over the sides.
Sprinkle half of the cheese evenly over the tortillas.
Spread the cooked chicken (or other protein), diced bell peppers, red onion, and seasonings evenly over the cheese.
Sprinkle the remaining cheese on top of the filling.
Place three more tortillas on top, overlapping to cover the filling, and fold the overhanging edges of the bottom tortillas over the top layer.
Lightly spray or brush the top with cooking spray or melted butter for a golden, crispy finish.
Place another sheet pan on top of the quesadillas and press down slightly to ensure even cooking.
Bake for 15-20 minutes until the tortillas are golden brown and crispy.
Remove from the oven, let cool for 5 minutes, then slice into squares and serve with salsa, guacamole, or sour cream.