Clean the spinach and wash it thoroughly. Wash salmon, dab dry and cut into 8 pieces. Heat 2 tablespoons of oil in a pan. Fry the salmon for 6-8 minutes at medium heat. Turn from time to time.
Season with salt and pepper. Meanwhile peel and finely dice the onion. Heat 2 tablespoons of oil in a saucepan. Add onion and spinach. Add 50 ml water, bring to the boil and stew the spinach for about 3 minutes. Melt butter over low heat. Bring to the boil briefly, skim off the foam. Put the egg yolk and white wine in a metal bowl and beat until frothy on a hot water bath. Add butter drop by drop while stirring constantly. Season sauce with lemon juice or zest, salt and pepper.
Bring to the boil briefly, skim off the foam. Put the egg yolk and white wine in a metal bowl and beat until frothy on a hot water bath. Add butter drop by drop while stirring constantly. Season sauce with lemon juice or zest, salt and pepper. Serve immediately with salmon and spinach on plates. Garnish with chervil and lemon slices. Serve with boiled potatoes