Preheat the oven to 180C/gas 4. Put the butter and oil in a large, flame-proof casserole over a medium-low heat. Add all the vegetables, season well and sweat them gently for 10 minutes, until they begin to soften.
Season the gurnard with salt and pepper, then add it to the pan, pushing it down so it is snuggled in among the vegetables.
Sprinkle over the wine, plus a glass of water, tuck in the bay leaves and cover the dish. Bring to a gentle simmer on the hob.
Transfer to the oven. Bake for 30-40 minutes, depending on the size of the fish. To check it is done, insert a knife at the thickest part and pull the flesh away from the bone. It should be completely opaque.
If you have one or two larger fish, take the flesh off the bones in big chunks. Otherwise, simply serve one fish per person, with plenty of the vegetables and juices alongside.