Heat 2 tbsp of the oil in a saucepan over a medium heat. Fry the onion with a pinch of salt for 7 mins. Add the garlic, chilli and rosemary, and cook for 1 min more. Tip in the tomatoes and sugar, and simmer for 20 mins.
Heat the remaining oil in a medium frying pan over a medium heat. Squeeze the sausagemeat from the skins and fry, breaking it up with a wooden spoon, for 5-7 mins until golden. Add to the sauce with the milk and lemon zest, then simmer for a further 5 mins. To freeze, leave to cool completely and transfer to large freezerproof bags.
Cook the pasta following pack instructions. Drain and toss with the sauce. Scatter over the parmesan and parsley leaves to serve.