First make the ragu or meat sauce. Heat the oil in a frying pan, then cook the beef in two batches for about 10 mins until browned all over. Finely chop 4 slices of prosciutto, then stir through the meat mixture.
Pour over the tomato sauce and stock, add the nutmeg, then season. Bring up to the boil, then simmer for 30 mins until the ragu looks rich and is well coated in sauce. Can be left for 3 days in the fridge or frozen for 3 months (although not if the tomato sauce has previously been frozen).
Heat oven to 180C/fan 160C/gas 4. To assemble lasagne, lightly oil an ovenproof serving dish (30 x 20cm). Spoon over a third of the ragu sauce, then cover with lasagne sheets. Drizzle over about one quarter of the white sauce. Repeat until you have 3 layers of pasta. Cover with the remaining half quantity of white sauce, making sure you can’t see any pasta poking through.
Tear the mozzarella into thin strips, then scatter over the top. Arrange the rest of the prosciutto on top. Bake for 45 mins until the top is bubbling and lightly browned. (The uncooked lasagne can be frozen. If baking from frozen, add another 45 mins to the cooking time).