Preheat the oven to Gas Mark 5, 190°C, fan 170°C. Heat the butter in a medium saucepan. Add the leeks and thyme, plus a splash of water, then cover and leave to cook gently for 10 minutes, until the leeks are soft.
Cool for a couple of minutes, then stir in the ham and ricotta.
Spread a heaped tablespoon of the mixture across the shorter end of each lasagne sheet, then roll up. Put in a large, greased baking dish so they fit snugly.
Stir 100ml (3½fl oz) water into the cheese sauce then pour over the lasagne. Scatter with the grated cheese.
Bake in the oven for 20 minutes until golden and bubbling. Serve with some green salad, if you like.
Freezing and defrosting guidelines
Once the dish has cooled completely, transfer it to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. Loosely cover with foil and bake until dish is thoroughly heated through. Reheat until piping hot.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.