Start by cutting the butternut squash in half, remove the seeds and drizzle over a little oil. Season with salt and pepper, rosemary and roast with the skin on in the oven for 30 minutes. Meanwhile, saute together the butter, onion and garlic until soft. Add in the rice and cook for 2 minutes before splashing in the wine to evaporate. Add in the saffron strands and season well with salt and pepper. Scoop out the flesh of the cooked butternut squash and add to the rice, gradually add in the stock and cook slowly. When almost cooked, add in the cream and cook for a further 4 minutes. Remove off the heat, stir in the shredded mozzarella and serve in the butternut squash skins.