Preheat the oven to gas 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.