In a wide skillet, heat oil or butter over medium-high heat. Add mushrooms and spread in a single layer. Cook undisturbed until browned, three to five minutes.

Stir and add onions, red pepper, corn, salt, garlic powder, onion powder, and black pepper. Cook until onions soften and smell sweet, about five minutes. Remove vegetables from the skillet and set aside.

Wipe the skillet clean and turn heat to medium. Lightly spray or brush with additional oil and lay a tortilla inside. Spread a quarter of the shredded cheese over the entire tortilla. Top half the tortilla with a quarter of the cooked vegetables. Carefully fold the tortilla in half, using a spatula to press down so it stays folded.

Cook until toasted, about two minutes. Carefully flip and cook until the second side is toasted and cheese is melted, one to two minutes more. Transfer to a cutting board and cut in half. Repeat with remaining tortillas, cheese, and cooked vegetables, and serve with salsa, guacamole, and sour cream.