Learn how to bake salmon in the oven with mayonnaise and onions in simple steps. You will need: a large onion, salmon, mayonnaise1. First peel out the outer layer of the onion, then cut it out and separate each of its layers. 2. Now slice the separated layers of the onion nicely and keep it aside. 3. Now take a suitable tray, place a piece of aluminum foil over it and spray it with butter. 4. Place the cleanly rinsed salmon on the tray and season it. 5. Add some oil to a pan, then add the sliced onions and cook them for a few minutes. 6. Now spread a thin layer of mayonnaise on top of the salmon and then add the onion over it. 7. Finally bake it in a preheated oven for about 45 minutes and serve hot.