Wash the fish, dab dry and cut lengthwise in the middle. Sprinkle with lemon juice and season with salt and pepper. Wash the dill, dab dry and chop finely. Spread on the fish.
Place a slice of bacon on each. Roll up the fish and pin it with a wooden skewer. Heat 250 ml of water lukewarm and stir in the sauce powder, bring to the boil. Season to taste with horseradish. Pour sauce into a small casserole dish.
Place fish rolls in the sauce. Wash and clean the tomatoes, cut them in half and spread around the fish rolls. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for 15-20 minutes. In the meantime, bring 1/2 litre of water to the boil and pour into a bowl.
Stir in the puree flakes with a whisk and let it swell for about 1 minute. Add herbs and stir in. Serve fish gratin to taste, garnished with dill and dill flowers. Add the herbs and mashed potatoes separately.