In a pan gently sweat your vegetables and garlic in butter, just to start the softening process, but not so that they have given up all resistance. Season. Lay them as a bed for your fish in your dish/oven pan. Place the fish on its vegetable bed, dot with knobs of butter and season, then pop it into a hot oven. Check the fish with a knife—when done the flesh should part easily from the bone. Keep a very close eye after 10 minutes.