Peel the onions. Heat 1 teaspoon of oil in a saucepan and sauté whole onions for 2-3 minutes. Add water so that the onions are covered and simmer covered for 8-10 minutes. Drain onions.
Prepare rice in boiling salted water according to package instructions.
Clean and trim the mushrooms and, depending on size, halve or quarter them. Wash the fish, dab dry and cut into cubes. Heat 2 tablespoons of oil in a frying pan. Fry the mushrooms for about 5 minutes, turning them until golden brown.
Take out the mushrooms and fry the fish for about 5 minutes, turning them over. After about 2 minutes add the onions. Season with salt and pepper.
Melt the fat in a pot. Stir in flour and sweat for 1-2 minutes. Slowly stir in white wine and milk and let simmer for 6-8 minutes while stirring occasionally. Add mustard and season to taste with salt, pepper and some lemon juice.
Wash the chervil, shake dry and chop the leaves of 5 stems finely. Add salmon, mushrooms, onions and chopped chervil to the sauce. Arrange fricassee and rice in bowls and garnish with chervil.