Heat olive oil in a skillet over medium heat.
Add sliced mushrooms to the skillet and sauté until browned.
Stir in the chopped spinach and garlic powder, and cook until wilted.
Season the mixture with salt and pepper to taste, then remove from heat.
Lay a tortilla in the skillet and sprinkle half with shredded cheese, then top with the mushroom and spinach mixture.
Fold the tortilla over and cook until golden brown and the cheese melts, about 3-4 minutes per side.
Remove the quesadilla from the skillet, slice into wedges, and serve immediately.