Scrub and debeard the mussels and, with your thumb and forefinger, try to open the shell of each one. If it doesn’t open that’s perfect. If it does open you will find it is full of very, very fine sand which will completely ruin the dish, so you must check each mussel and throw away any open ones. Put the mussels into a large pan, add the white wine (no other liquid), cover with a tight-fitting lid and cook the mussels over a medium heat until the shells have opened. Strain the mussels and reserve the resulting juices. Remove the mussels from their shells, discarding any that haven’t opened, and put to one side. Melt the butter in a pan, stir in the flour and cook for a moment or two to make a roux. Make the reserved mussel-cooking juices up to 300ml/½ pint with milk if necessary and whisk them into the roux. Bring to the boil, whisking, until you have a smooth sauce. Finish it off by stirring in the cream. Season with grated nutmeg and black pepper.
Mix the spinach and mussels together in a shallow gratin dish. Pour over the white sauce, sprinkle the cheese over the top and grill until golden brown.
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