Heat oil in a nonstick skillet over medium heat. If using onions, sauté 3-5 minutes, or until soft and translucent. Add remaining vegetables. Continue cooking until all of the vegetables are soft.
Preheat oven to 350°.
Sprinkle 1/3 cup of cheese and 1/3 cup of vegetables evenly over each tortilla; fold in half. Place tortillas on baking sheet lightly coated with cooking spray. Bake for 10 minutes, or until cheese melts. Enjoy with your favorite salsa.
Variation: Add a tablespoon of cooked black or pinto beans to the vegetables in each quesadilla.