Peel and wash the potatoes, cover and cook in salted water for about 20 minutes. Clean, wash and cut leek into rings. Rinse salmon fillet, dab dry and cut into coarse cubes. Finely dice smoked salmon.
Wash the dill, shake dry, pluck flags from the stems and cut finely.
Heat the oil in a large frying pan. Fry the fresh salmon fillet cubes in it for 4-5 minutes all around, take them out. Sauté the leek in the frying fat for about 3 minutes. Dust with flour and briefly sauté lightly.
Add 200 ml milk while stirring, bring to the boil and simmer for about 2 minutes. Season sauce with salt, pepper and lemon juice. Stir in dill.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Grease ovenproof dish. Carefully fold the smoked and fried salmon into the leek. Pour the salmon-porree-mix into the baking dish.
Drain the potatoes. Add 150 ml milk, mustard and 2 tablespoons butter. Mash with a potato masher until puree. Season to taste with salt and pepper. Spread on the salmon. Bake in a hot oven for about 30 minutes.