large white onion • white filet of skinless seafish (cod, halibut or such) • small pinch salt and ground pepper each • softened butter • mustard with grains • hot mustard of choice • grated edam, gouda or similar cheese • breadcrumbs • mixed herbs, (parsley, chives, coriander) • hardboiled eggs, optional • thinly sliced tomatoes, optional • lettuce of choice, optional •