Heat the butter in a water bath so that it is very soft but not runny. Mix in the herbs, mustard seeds and mustard. Rub the zest of the lemon and add to the butter. Season the butter with salt, pepper, sugar and lemon juice and set aside (not in the refrigerator).
Clean the fish fillets. At the front of the belly flap there could be 2 very thick bones, remove them. Otherwise the fish is boneless. Then drizzle the fillets with lemon juice and season with salt.
Lay out a sheet of aluminum foil for each portion, place the fillets on top and brush with the butter. Close the foil tightly on all sides and place in the oven at 180 ° C for about 30 minutes.
Serve the package closed. Cucumber salad and boiled potatoes go well with it.