Rub the butternut with about 1 tablespoon of melted butter and sprinkle with salt.
Roast the butternut with the cut-side up on a baking tray for about an hour at 190°C. (You will know that it is ready when a butter knife goes into it with ease)
While the butternut is roasting, sauté sliced onion in a tablespoon of butter and when it is soft, add the brown sugar and cook for about 3 minutes. Remove from heat.
When the butternut is roasted, scoop the flesh out with a spoon and put it into a blender.