Wash the potatoes and cook in boiling water for 15 to 20 minutes. In the meantime, heat the fat in a pot and sauté the flour in it. Add stock, milk and cream while stirring. Let the sauce simmer for five minutes, season with mustard, salt and pepper and keep warm. Drain the potatoes, rinse with cold water and peel.
Cut potatoes in half and add to the hot sauce. Wash the dill, dab dry and cut finely, except for a little to garnish. Dice salmon fillet. Place the béchamel potatoes in a baking dish and sprinkle with salmon cubes and dill. Serve garnished with dill flags