Bring 2 l salted water to the boil. Peel potatoes with a potato peeler, wash and cut into quarters or halves depending on size. Cover and cook in boiling water for about 25 minutes.
For the sauce, melt butter in a pot at low heat (but do not brown!). Add the flour completely and sweat with a whisk while stirring for about 1 minute (must not get dark).
Add the milk little by little. Bring the sauce to the boil while stirring, simmer for about 5 minutes and remove from the heat. Roll the lemon, cut it in half, squeeze one half and stir 1-2 tsp. into the sauce. Season with salt and pepper.
Wash parsley, shake dry, pluck off leaves, chop finely and stir into the sauce. Keep the sauce warm in the pot.
Rinse the salmon, dab dry with kitchen paper. Sprinkle the juice of the other half of the lemon over the salmon. Heat the oil in a large coated pan. Sauté the fish in it for about 3 minutes until crispy on the skin side.
Season with sea salt and pepper. Turn with a wooden spoon and a spatula, fry again for 3-5 minutes and season.
Drain the potatoes. Warm up the sauce again briefly and serve with salmon and potatoes.