Sauté the sliced onion and mushrooms with oil until softened and slightly golden.
Place the salmon fillets on baking sheet. Season to taste with salt and pepper and rub the tops with mayonnaise.
Layer on the onions and mushrooms, followed by a layer of grated cheese. Bake in a preheated oven at 400°F for 15–18 minutes, or until the internal temperature reaches 145°F.
Serve immediately while the cheese is still melty and enjoy!