Heat the oil in a large skillet over medium-high heat. Add the onion, bell peppers, jalapeno pepper (if using), garlic and mushrooms. Cook until the vegetables are softened and lightly browned.
Add the beans, cumin and chili powder. Season with salt and pepper to taste. Cook for an additional 5 minutes.
Divide the mixture among 4-6 flour tortillas. Fold in half to form quesadillas. Place on a preheated griddle or skillet over medium heat. Cook until lightly browned on both sides.
Serve with your favorite toppings such as salsa, guacamole or sour cream.