Rinse the fish fillet, pat dry with kitchen paper, salt and pepper. Mix the Parmesan with breadcrumbs, butter and chopped dill.
Preheat the oven to 200°C. Boil the cream with broth – possibly vermouth – and mustard. Then stir in the cornstarch with a whisk and let it boil down a bit.
Stir in the parsley and season everything with salt and pepper.
Pour the mustard sauce into an ovenproof casserole dish, place the fish in it and coat evenly with the Parmesan mixture. Place the fish fillet in the hot oven and bake for about 30 minutes. If the Parmesan crust gets too dark, cover with aluminum foil.
Tip: As a variation, you can place sliced tomatoes on the fillet and then brush with some of the mustard sauce.