Peel, wash and halve the potatoes. Clean, wash and cut the leek into small pieces. Cook potatoes in little boiling salted water for about 15 minutes, leek for about 10 minutes. Wash the fish.
Let it simmer in the hot stock for about 10 minutes. Take out and keep warm for a short time. Stir the sour cream and mustard into the stock. Bring to the boil again and thicken with sauce thickener. Season to taste with salt, pepper and lemon juice.
Drain the vegetables. Wash, clean, quarter and seed the tomato. Finely dice the flesh. Drain potatoes and leek. Melt the fat. Swivel vegetables in it. Arrange fish, vegetables, potatoes and sauce on a plate.
Sprinkle diced tomatoes over it. Garnish with dill and lemon.