In a large skillet, heat the oil over medium high heat. Add the mushrooms, peppers and garlic. Cook, stirring constantly, for 1 to 2 minutes. Add the spinach and corn and cook just until the spinach is wilted, 1 or 2 minutes.
Spread half of the mixture onto one tortilla, sprinkle with the chopped tomatoes and half of the cheese. Place the second tortilla evenly over the mixture.
Melt the butter or oil in large skillet over medium high heat. Place the quesadilla in the skillet and cook for 2 to 3 minutes on each side or until golden brown and the cheese is melted. Repeat with the remaining ingredients.