1. Preheat the oven to 175°C. Chop the butternut squash into small pieces. Drizzle with a little olive oil, salt and pepper. Roast for 15 minutes, turning the pieces once.
2. Remove the squash from the oven. Let slightly cool. Place squash in a food processor, add the milk, and puree until smooth.
3. In a frying pan over high heat on the stove, add the butter and olive oil. Once hot, add the onion and garlic. Stir fry until soft.
4. Add the squash puree, nutmeg, parmesan cheese and salt and pepper to taste. If the sauce looks a bit thick, add a little more milk. If the sauce looks a bit runny, just continue to cook until it thickens.
5. Boil the cheese tortellini until ready in salted water. Strain the cheese tortellini and toss it together with the sauce. Serve with grated parmesan cheese on top.