Season the fish fillets with salt and pepper and place in a greased baking dish or on a baking sheet. Mix the butter with both types of mustard, cheese, onions, breadcrumbs and herbs to a paste. Spread the paste on the fillets and bake the fish in the preheated oven at 200 ° C top / bottom heat for 15-20 minutes.
Boiled potatoes or creamed potatoes are very good with it.