Heat the oil in a large skillet over medium heat. Add the onion and bell pepper and cook for about 5 minutes until softened.
Add the garlic, cumin, chili powder, salt and pepper. Cook for 1 minute until fragrant.
Add the chicken and cook for 5-7 minutes until cooked through.
To assemble the quesadillas: Place 4 of the tortillas on a work surface. Sprinkle 1/2 cup of cheese over each tortilla. Top with cooked chicken mixture and remaining cheese.
Top with remaining tortillas. Heat a large skillet over medium heat. Place one quesadilla in the pan and cook for 2-3 minutes per side until golden brown and cheese is melted.