Preheat the oven to 200°C/400F. Place the butternut and garlic on a baking tray and drizzle with olive oil, coating well. Season with salt and pepper and bake for 25 to 30 minutes or until caramelised and tender.
Blend the butternut, mascarpone and stock in a food processor or blender until smooth. Adjust the seasoning.
Heat a non-stick frying pan with a little olive oil and sauté the mushrooms until they just start to caramelise (about 4 minutes). Add the thyme and cook for another minute or two.
Add the puréed butternut mixture to the mushrooms and stir through.
Cook the pasta in salted water until al dente and drain, reserving some of the cooking liquid. Add the cooked pasta to the butternut sauce and stir. If the sauce is a bit thick, add about 60ml of the pasta water to thin it out.
Serve with freshly grated Parmesan, roasted pumpkin seeds, and sage leaves fried in butter.