Place the halved butternut squash into a deep roasting pan then season with the salt.
Pour the boiling water into the pan and cover with foil.
Place in the oven and allow to roast for 30-45 minutes until soft. Once roasted, scoop out the seeds and some of the flesh. Set aside.
While the butternut is roasting, make the creamed spinach. Saute the onion and garlic in 2 tsp of olive oil until soft and translucent.
Add the spinach and cook until wilted then add the flour and stir into the spinach. Pour in the milk and stir until the sauce is smooth then add the cream. Allow to simmer gently for 7-10 minutes or until the sauce is thick and smooth. Season to taste and remove from the heat. Allow to cool for 10-15 minutes before adding the mozzarella cheese.
Once the butternut is roasted, spoon the creamed spinach into the hollows of each butternut and top with Parmesan cheese.
Place back in the oven and allow to cook until the tops are golden brown, approximately 10 minutes.