Preheat oven to 425F. Seed and de-fibre the squash, then cut it into chunks and place it skin-side down in a heavy baking tray brushed with olive oil. (If you line the tray with foil, you'll save yourself a lot of effort on the washing-up.) Pour over 3 tbsp olive oil, and scatter the salt, pepper, herbs and garlic on top. Cover with foil and bake until soft and browning at the edges (about 50 minutes). Scrape the flesh from the skin and save the juices.
Heat the stock and check the seasoning. Heat 3 oz butter and the rest of the oil in a heavy pan; gently fry the onion for about 15 minutes, until soft. Remove from the heat, add the rice and stir quickly until the rice is coated. Return to the heat and add 2 ladles of stock, simmering and stirring until it is nearly all absorbed. Add more stock and repeat. After 15-20 minutes, almost all the stock will be used and the rice will be creamy but still al dente. Add the remaining butter, squash, vermouth and parmesan. Stir gently and serve immediately.
From the Dinner Co-op