Fry the onion with a pinch of salt for 10-15 mins until soft and translucent. Add garlic and fry for 1 minute more. Remove from the heat and leave to cool.
Cook the pumpkin in boiling salted water for 10-15 minutes until tender. Blitz with 50ml milk and onions until smooth, gradually adding more milk until the mixture is thick enough to just coat the back of a spoon. Pour into a large frying pan with the tomato purée and mascarpone, and simmer over low heat.
Cook your pasta and drain, reserving a cupful of the cooking water. Toss the pasta with the pumpkin sauce, parmesan and 50-100ml of the reserved water to loosen. Season with extra parmesan. Enjoy!