Cut the tomatoes into slices and place in a baking dish. Salt and pepper. Place the fish fillet on the tomato slices, season with salt, pepper and drizzle with a little lemon juice.
Puree the basil or other herbs with the oil, mustard and stock.
Mash the boiled potatoes and mix with the sour cream and grated cheese.
Mix the potato and herb paste and season.
Spread on the fish and place in the oven for 25-30 minutes at 200 ° C.