Heat oven to 180°C/ gas 6. Peel the squash, divide in half and cut into 2cm cubes. Toss in olive oil, season with salt and pepper and roast in the oven until golden brown and soft in the middle. This will take about 30 minutes. Warm the vegetable stock and leave aside.
Heat a medium sized frying pan while the squash is roasting. Add the rest of the olive oil and half of the butter. Toss in the onion and cook for 3 minutes until softened. Mix in the celery, garlic, herbs and a little pepper. Cover the pan and cook for about 2 minutes. Turn the heat up slightly and mix in the rice. Stir continuously for about 5 minutes.
Increase the heat, stir in the wine and allow to simmer. Reduce the heat and begin adding the stock gradually. Stir gently between adding large spoonfuls of stock. This will help the rice absorb the liquid. Wait until the stock has been absorbed before adding another spoon/ladleful. This will take about 15 minutes before all the stock has been added.
Remove from the heat; add the rest of the butter and the parmesan to the risotto. Cover for 2 minutes and then stir. Transfer the risotto to shallow bowls and top with the roasted squash and remaining parmesan.