Knead the ingredients for the herb crust, shape into a 3 cm thick roll, wrap in cling film and allow to set in the refrigerator.
Preheat the grill. Season the fish on both sides with salt and pepper, fry on both sides in a large pan, place side by side in a greased casserole dish.
Spread thinly with mustard. Cut the butter roll into thin slices and cover the fish with it. Gratinate until golden brown under the grill.