Prepare spinach according to package directions. Drain and pat dry.
Preheat oven to 350 degrees F (175 degrees C). Sprinkle 1/4 cup cheese on one side of each tortilla. Place tortillas cheese side up on baking sheets, and bake 5 minutes, or until cheese is melted.
Melt the butter in a skillet over medium heat. Stir in garlic and mushrooms, and cook about 5 minutes. Mix in spinach, and continue cooking 5 minutes. Place an equal amount of the mixture on the cheese side of each tortilla. Fold tortillas in half over the filling.
Heat a separate skillet over medium heat. Spray quesadillas with veggie spray Place quesadillas in the skillet one at a time, and cook 3 minutes on each side, until golden brown. Cut each quesadilla into 4 wedges to serve.