Preheat the oven to 180 ° C. Peel and finely dice the onions. Grate the mushrooms and cut them into leaves. Rinse fillets, pat dry and sour.
Heat some butter in a pan and fry the onions until translucent. Add the mushrooms and fry briefly. Mix the mushroom mixture and parsley with the tomato sauce.
Grease a mold and add the pollack fillets, season with salt and pepper. Whisk the wine and cream together and pour over the fish. Spread the mushroom mixture over the fish. Sprinkle with the breadcrumbs and top with flakes of butter. Bake in the oven for 30 to 40 minutes.