Preheat the oven to 400°F (200°C). Line a sheet pan with parchment paper if desired, and lightly spray it with cooking spray or brush with olive oil.
In a mixing bowl, combine the cooked chicken (or black beans), bell peppers, onions, cumin, chili powder, salt, and pepper. Mix well to combine all ingredients.
Place 4 tortillas flat on the sheet pan. Evenly distribute half of the cheese mixture over the tortillas.
Spread the chicken and vegetable mixture evenly on top of the cheese layer.
Top with the remaining cheese, and cover with the remaining 4 tortillas.
Brush the tops of the quesadillas with olive oil or spray lightly with cooking spray.
Bake in the preheated oven for 15 minutes or until the tortillas are golden brown and the cheese is melted.
Remove from the oven and let the quesadillas cool for a few minutes. Cut into wedges with a sharp knife or pizza cutter before serving.
Serve with salsa, sour cream, or guacamole for dipping.