Step 1

Preheat the oven to 400 degrees Fahrenheit. Line a rimmed baking sheet with aluminum foil.

Step 2

Place the salmon fillet on the prepared baking sheet. Season with salt and pepper. Layer the thinly sliced onion on top of the fish, so it covers the surface of the fish completely.

Step 3

In a small bowl, combine the mayonnaise, lemon juice, mustard and garlic. The mixture should be thin enough to easily spread over the onions. Cover the surface of the onions with the creamy mayo mixture and spread it out evenly. Top with the finely grated cheese.

Step 4

Bake in the preheated oven for 15-20 minutes,then broil for 2-5 minutes, until the cheese is golden brown.