Heat the oil in a large frying pan, and fry the onion for 5 mins. Once starting to soften, add the sausagemeat and chestnuts, and fry for another 10-12 mins, breaking the meat up with a wooden spatula or spoon as you fry it. Keep the heat quite high and stir often.
When the meat and chestnuts are golden brown, add the garlic, rosemary and fennel seeds, and cook for 2 mins more. Splash in the wine, let most of it boil away, then tip in the passata. Cover and simmer the sauce for 10 mins while you boil the pasta in plenty of salted water, following pack instructions.
Reserve a cup of the pasta cooking water, then drain the pasta well. Toss the pasta, 3 tbsp of the water, the ragu and most of the parsley together, then season to taste. Share between bowls, add a grinding of black pepper, the remaining parsley, then a handful of Parmesan shavings.