Rinse the fillets briefly and pat dry. Depending on the size, cut into 2 - 3 pieces, season with salt and pepper and place in a greased baking dish. Preheat the oven to 200 ° C top / bottom heat.
Heat the butter in a saucepan and sauté the onions, cut into fine rings, for about 15 minutes. The onion rings can brown a little, that refines the taste. Then dust with the flour, sweat a little and season with salt and pepper. Deglaze with the vegetable stock and cream and let the sauce simmer a little.
Pour the sauce over the fillets. If you like, you can sprinkle some grated cheese on top. Put the mold in the oven for 30 minutes.