Heat oven to 220C/fan 200C/gas 7. Heat the oil in a shallow frying pan or casserole dish, one that can go in the oven. Add the onion, garlic, rosemary and a pinch of salt, then cook for a few mins until softened. Squeeze the sausagemeat from their skins into the pan. Break it up with your spoon and cook until starting to brown, around 3-5 mins.
Tear the red peppers into the pan and add the fennel seeds, chilli (if using), passata, sugar and some seasoning. Cook over a medium heat for 5 mins. Meanwhile, mix the mascarpone, milk, 50g parmesan and some seasoning in a bowl.
Stir the basil into a pan, then remove from the heat. Spoon half the ragu into another pan or bowl. Put a layer of lasagne sheets on top of the ragu in the casserole dish, then top with half the mascarpone mixture. Add another layer of ragu, followed by the pasta and mascarpone. Top with the remaining parmesan, then put in the oven on the middle shelf and bake for 5-10 mins until golden and bubbling.