In a large nonstick pan, cook red pepper, zucchini, yellow squash, onion and mushrooms in olive oil over medium to medium-high heat for about 7 minutes, or until just tender. Remove vegetables from pan.
Coat the same pan with cooking spray, and place one tortilla in pan.
Sprinkle ¼ cup cheese evenly over tortilla, and layer ¾ cup of the vegetable mixture over the cheese.
Sprinkle another 1/8 cup of cheese on the vegetables, and top with a second tortilla.
Cook until golden on both sides, for approximately 2 to 3 minutes per side.
Remove quesadilla from pan, and repeat with remaining ingredients.
Cut each quesadilla into 8 triangles with a pizza cutter.