Wash the potatoes thoroughly. Cook in boiling water for about 20 minutes. Clean, wash and slice the leek. Cook in slightly salted water for about 5 minutes. Pour off the cooking water and catch it.
Melt the fat in a pot. Stir in flour and sweat briefly. Add 1/2 litre vegetable water and milk bit by bit while stirring. Bring to the boil, boil for 1-2 minutes and season with horseradish, salt and pepper.
Drain the potatoes, quench, peel and slice. Add potatoes and leek to the sauce. Season again with salt and pepper. Wash salmon, dab dry, cut into 4 equal pieces. Season with salt, pepper and lemon juice.
Heat clarified butter in a pan and fry salmon on each side for about 5 minutes. Wash the dill, dab dry and cut. Arrange salmon and béchamel potatoes on plates. Sprinkle with dill and garnish with lemon wedges and dill flags.