Sauce

• 30 g(1 oz) butter
• 30 g(1 oz) plain (all-purpose) flour
• 300 ml(10½ fl oz) vegetable broth, warmed
• 100 ml(3½ fl oz) pouring (single) cream
• 60 g(2 oz) butter
• 1 garlic clove, peeled and squashed a bit
• 200 g(7 oz) button mushrooms, sliced
• 2 tablespoons white wine
• 500 g(1 lb2 oz) skinless halibut, cod or snapper fillets
• 500 g(1 lb2 oz) raw prawns (shrimp), peeled, deveined
• 2 tablespoons chopped parsley
• 1 kg(2 lb4 oz) potatoes, peeled and cut into chunks
• 185 ml(6 fl oz/¾ cup) milk, warmed
• lemon wedges, to serve

For the sauce, melt the butter in a smallish saucepan and then stir in the flour. Cook for a minute or so over medium heat and then add the broth, whisking well to make a smooth sauce. Let it bubble up for a few minutes and then stir in the cream. Taste for salt and pepper (your broth will probably be seasoned already).

Preheat the oven to 200°C (400°F/Gas 6) and butter a 32 × 22 × 6 cm(13 × 9 × 2½ inch) ovenproof dish. Heat 20 g(¾ oz) of the butter and the garlic in a frying pan and, when it is sizzling, add the mushrooms and a little salt. Sauté over high heat, shifting the mushrooms around with a wooden spoon until all the juices have evaporated and they turn golden. Add the wine and some pepper and cook until the wine has evaporated. Remove from the heat.

Cut the fish into large chunks of about 5–6 cm(2 inches) and put in the bottom of your dish. Scrape the mushrooms out over the fish, then add the prawns. Scatter with the parsley and a few grinds of pepper. Pour the sauce over the top. Bake for about 20 minutes or until the sauce is bubbling and a bit golden here and there.

Meanwhile, boil the potatoes in salted water until soft. Heat the remaining butter in the milk. Drain the potatoes and mash them or pass them through a potato mill into a bowl. Beat in the rest of the butter and the warm milk and season, if necessary.

Arrange the mash in dollops just around the rim of the dish, then bake the pie for another 10 minutes or so, or until the mash is a little golden in places. Leave it to stand for 5 or 10 minutes before serving with lemon wedges.