Thoroughly brush the potatoes and cook with the skin in plenty of boiling water for 15-20 minutes. In the meantime, wash the salmon chops, pat dry and sprinkle with 2 tablespoons of lemon juice and salt.
Peel and finely chop the onion. Heat 30 g fat in a pot and fry half of the onions in it. Add flour and sauté while stirring. Then gradually pour on the stock and milk while stirring.
Bring the sauce to the boil and simmer for about 5 minutes. Carefully wash the dill and chervil and dab dry. Chop the herbs, except for something to garnish. Stir the herbs, except for a tablespoon for sprinkling, into the sauce and season to taste with salt, pepper, nutmeg and the remaining lemon juice.
Pour the potatoes onto a sieve, rinse under cold water, drain and peel. Add potatoes to the hot sauce. Clean, wash and drain the salad and pluck into bite-sized pieces.
Mix vinegar, remaining onions, mustard, some salt and pepper. Whip the oil into the mixture. Mix the vinaigrette with the salad in a bowl. Heat the remaining fat in a frying pan. Dab salmon chops dry and fry on each side for about 3 minutes.
Arrange béchamel potatoes and salmon chops on a plate. Sprinkle with remaining herbs. Serve garnished with a slice of lemon and herbs.