Fry the onions on low heat in olive oil until they become translucent and nicely golden. Add the chopped mushrooms and cook until cooked. The mass will no longer be watery, the oil will separate easily when you stroke it with a spatula. Cool down.
Place the salmon in the baking dish. Choose the size as close as possible to the fish. I shortened the thin end of the tail and tucked it in, so the whole piece of salmon was of a similar thickness and cooked evenly. Pour olive oil, it should almost cover the fish, 100 ml was enough for me, but the amount depends a lot on the size of the container. Season with salt, sprinkle with freshly ground pepper and cover with mushrooms and onions.
Bake, or rather this method is called confit, at a low temperature of 100 degrees without a breeze, for half an hour. If you want an even pinker salmon, reduce the temperature to 80 degrees.
