Preheat the oven to 400 degrees F (200 degrees C).
Line a rimmed baking sheet with aluminum foil. Place the Salmon Fillet (2 pound) on the prepared baking sheet.
Season with salt and pepper. Season liberally, since the fish is very thick and the salt needs to penetrate all the way through to the center.
In a small bowl, combine the Mayonnaise (1/4 cup), Lemon Juice (2 tablespoon), Dijon Mustard (1/2 tablespoon), and Garlic (1 clove). The mixture should be thin enough to easily spread over the onions.
Cover the surface of the fish with the Onion (1) and creamy mayo mixture and spread it out evenly.
Top with the Parmesan Cheese (1/4 cup) and Mozzarella Cheese (1/3 cup).
Bake in the preheated oven for 15-18 minutes.
Then broil for 2-5 minutes, until the cheese is golden brown.