1. To make the topping,cook the potatoes and garlic in simmering salted water for 20 minutes until tender. Drain the potatoes well and mash with butter,milk, parmesan,sea salt and pepper.
2. To make the sauce,melt the butter in a pan,add the flour and stir well with a wooden spoon over low heat for one minute,without allowing to brown. Slowly add the milk,stirring constantly,then the stock and simmer for five minutes. Add mustard,cream,paprika,cayenne,sea salt and pepper and simmer for one minute.
3. Heat the oven to 220C. Chop the fish and prawns into bite-sized chunks. Gently fold the fish and prawns into the sauce with the parsley and chives. Simmer for three minutes,then transfer to a large,buttered oven-proof gratin dish.
4. Cover the top with tablespoonfuls of mash,scatter with cheese and breadcrumbs, place on a tray and bake for 25 minutes or until golden and bubbling.
Serve with a butterybraise of peas,leeks and bacon for extra warmth.