Tip the milk, nutmeg, 50g of the butter and the flour into a non-stick pan. Heat while whisking or stirring until thickened. Remove from the heat and stir in the cream with plenty of seasoning. Measure
450ml of the sauce and beat half the Parmesan and the egg into it. Once cool, stir the crabmeat into the remaining sauce.
Melt the remaining butter in a very large pan or wok, add the leeks and cook for about 10 mins, stirring frequently, until softened and cooked down.
Lightly grease 2 x 20cm square ovenproof dishes – trim the lasagne sheets to fit, if you need to.
Spoon a quarter of the leeks into each dish and spread out. Top each with 2 lasagne sheets, then cover with the crab mixture and scatter over the chunks of fish. Top each with another 2 sheets, then cover with the leeks and the last of the sheets followed by the cheese sauce, then the remaining Parmesan.
To freeze: Wrap the dishes in several layers of cling film, then foil. Use within 2 months. Thaw overnight in the fridge. Heat oven to 180C/160C fan/gas 4. Unwrap the lasagnes and bake for 40 mins. (A freshly made version will take the same time.) Serve with a salad.