Haddock, spinach and Gruyère gratin

Method

1.  Melt 30g of the butter in a non-stick pan, add the flour and cook over a medium heat for 1 minute. Remove from the heat and gradually beat in the milk. Bring to the boil, stirring, and simmer very gently for 10 minutes, stirring occasionally.
2.  Meanwhile, preheat the grill to high. Melt another 15g of the butter in a large pan. Add the spinach and cook over a high heat, stirring, until wilted. Tip it into a colander and gently press out the excess liquid. Return to the pan with another 15g butter, season and cook over a high heat, stirring, for 1 minute. Remove from the heat and cover with a lid to keep hot.
3.  Put the remaining 15g butter into a shallow gratin dish and slide it under the grill until melted. Add the pieces of fish, turn them in the melted butter, then season. Place the dish back under the grill, about 10cm from the heat, and grill the fish for 4 minutes, until half cooked. Lift the fish onto a plate. Leave the grill on. Pour any juices into the simmering sauce. Spoon the spinach over the base of the dish and put the fish on top.
4. Stir 100g of the cheese and the crème fraîche into the sauce and season to taste. Pour the sauce over the fish and sprinkle with the remaining cheese and cayenne pepper, to taste. Slide the dish back under the grill for a further 4-5 minutes, until the gratin is golden brown and bubbling. Serve with crusty fresh bread, if you like.