Meanwhile, melt 30g of butter in a small pan and stir in 3 tbsp of flour, 1 tsp of English mustard and a pinch of nutmeg until well blended. Cook for 2 minutes. Remove from heat and gradually blend in 2 cups of milk until smooth. Return to the heat and bring to the boil, stirring until thickened. Gradually stir in 30 grams of grated cheddar cheese and 30 grams of grated parmesan cheese until melted. Season to taste.
Spoon half the meat mixture into the base of an ovenproof dish. Top with a single layer of precooked lasagne sheets. Spread over half the white sauce, and scatter more parmesan and grated cheese. Repeat the layers finishing with the cheese. Bake in a preheated oven, at 200 degrees for 30 minutes, or until the pasta is cooked the top is golden brown and bubbly.