Parmesan cheese 50g/2oz,
or vegetarian hard cheese, grated
Tip the pumpkin into a medium-sized saucepan that has a tight-fitting lid. Sprinkle over the sugar and a generous pinch of salt, then drizzle over 6 tablespoons of water. Cover the pan, place on a medium heat and steam the pumpkin, stirring every so often for 10-15 minutes, or until it is soft but still holds its shape. You may need to add a spoonful or two of water during the cooking if the pan seems dry. Set aside.
While the pumpkin is steaming, tip the butter and sage into a small saucepan and heat gently until the butter is foaming, then turn on the heat. Boil the spaghetti in a big pan of salted water for about 10 minutes until just cooked. When the spaghetti is cooked, scoop out a little of the cooking water, then drain and return the spaghetti to the pan.
Put the sage butter over a high heat until sizzling, then pour in the lemon juice and let it splutter for a second. Tip the pumpkin, melted butter, 3-4 tbsp pasta water and half the parmesan in with the spaghetti and give it a really good stir. Season generously with salt and pepper and serve with the remaining parmesan to sprinkle over.