Heat oven to 130C/110C fan/gas 1. Heat the oil in a casserole dish. Season the lamb generously, then sizzle for 5 mins until nicely browned. Add the garlic and rosemary and cook for a minute more, then pour over the passata. Rinse out the passata carton with a splash of water, then pour that in too. Season, then bring to a simmer. Cover tightly with foil, top with the lid, then cook in the oven for at least 3½ hrs or up to 4 hrs, until the lamb is shreddable. Leave the lamb to cool slightly, then use two forks to shred it into the sauce. Will keep in the fridge for two days, or in the freezer for 6 months.
To make the béchamel, heat the milk in a saucepan until just simmering. In a separate pan, melt the butter, then add the flour and stir to make a sandy paste. Slowly whisk in the hot milk until combined and you have a glossy white sauce. Stir in half the parmesan and season. Meanwhile, soak the lasagne sheets in just-boiled water to soften, then drain.
Heat oven to 180C/160C fan/gas 4. Spread a thin layer of the lamb sauce over the base of a small lasagne dish, then top with a layer of pasta, followed by a third of the béchamel and a third of the mozzarella. Add a second layer of lamb, then again top with pasta, then béchamel, then mozzarella. Add a final layer of pasta, then top with the rest of the béchamel, the last of the mozzarella, the remaining parmesan and the panko breadcrumbs. Bake the lasagne for 35 mins, then turn the oven up to 190C/170C fan/gas 5. Bake for 10 mins more to crisp up the top, then leave to rest for 10 mins before serving straight from the dish with a few dressed chicory leaves, if you like.