Finely dice the onions. Rinse the fish fillets under cold running water, pat dry, sprinkle with salt and pepper and place on a greased baking sheet.
Mix the butter with the two types of mustard, cheese, onions, breadcrumbs and herbs to a paste. Spread the paste on the fish fillets and bake at 180 ° C for about 20-30 minutes. A mixed salad with baguette tastes good with it.