Peel and dice the onion. Clean the mushrooms and cut them into leaves. Leave the ham in a pan, add the onion and mushrooms and sauté gently.
Wash the fish fillets, pat dry and drizzle with a little lemon juice. Season lightly with salt and pepper. Brush the fillets with the anchovy paste and tomato paste and place in a lightly greased baking dish. Spread the mushroom mixture over it. Put on flakes of butter, sprinkle with cheese and breadcrumbs.
Bake at 180 ° C (top / bottom heat) for about 20 minutes. Then spread the sour cream over the fish and bake for another 10 minutes.