Preheat your oven to 180˚C. Make your cheese sauce first by melting butter and stirring in flour to form a roux. Then pour in white wine followed by cream. Stir until it thickens into a glossy bechamel. Next add your cheese and mustard plus a pinch of nutmeg. Melt the cheese then remove from the heat and incorporate one egg yolk.
Place your fish in a casserole dish and cover with the sauce. Bake for 15 mins until the cheese is golden and the plaice is flaky.
While the fish is baking fry your pancetta in drizzle of oil and add in your savoy to wilt. Season with pepper and nutmeg. It’ll be salty enough with the cured pancetta and salty cheese. Add a splash of water. Serve hot and season to taste.