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Pour the milk into a large pan and add the flour and butter. Set over a medium heat and whisk continuously until you have a smooth, thick white sauce. Remove from the heat, add the mustard, most of the cheese (save a handful for the top), peas and parsley.
Meanwhile, boil the pasta in a large pan of water following pack instructions until just cooked. Drain.
Heat the oven to 200C/180C fan/gas 6. Tip the pasta into the sauce and add half the fish, stir everything together then tip into a large baking dish. Top with the rest of the fish, pushing it into the pasta a little, then scatter with the remaining cheese. Bake for 30 mins until golden, then serve with salad, if you like. Can be chilled and eaten within three days or frozen for up to a month. Defrost in the fridge, then reheat in a microwave or oven until piping hot.