Peel, wash and halve the potatoes. Cook in salted water for about 20 minutes. Drizzle the fish fillets with lemon juice, season with salt and place on a baking tray lined with baking paper. Drain and mash the potatoes. Stir in 1 egg, mustard, milk and 1-2 tablespoon butter and season with salt, pepper and nutmeg. Fill into a piping bag.
Sprinkle the puree in a spiral on each portion of the fish. Bake everything in a preheated oven at 200 degrees for 20-25 minutes.
Whisk the egg yolk with the condensed milk and brush the puree with it after baking for 10 minutes.