Peel, wash and halve the potatoes. Cook in salted water for about 20 minutes
Clean, wash and cut the leek into rings. Fish wash, dab dry and cut into 8 pieces (tranches). Sprinkle with lemon juice
Drain the potatoes and press them through a potato ricer. Mix whole egg and horseradish. Stir milk and 1-2 tbsp. butter into the puree. Season with salt and nutmeg. Pour the puree into a piping bag with a large star-shaped spout
Season fish with salt and place on a baking tray lined with baking paper. Spray the puree onto it in a spiral shape. Bake in a preheated oven (electric cooker: 200 °C/recirculating air: 175 °C/ gas: level 3) for 20-25 minutes until golden brown
Wash and halve the tomatoes or cut off a lid. Season with salt and pepper. Sprinkle with cheese and spread 1-2 tsp. butter in flakes on top. Place the tomatoes on the tray after about 10 minutes. Whisk egg yolk and cream. Spread the potato crust with it and bake until done
Fry the leek in 2 tablespoons of hot butter. Pour on 100 ml water, cover and steam for about 12 minutes. Season to taste with salt and pepper. Arrange and garnish everything