Preheat oven to 425. Halve and seed the butternut squash and place it on a baking sheet and brush with some o the oil and roast for 25 minutes. Reduce the temp to 375 and cook for 20-25 minutes more or until its tender.
Heat the remaining oil in a large heavy pan and cook the chopped onions over a low heat for about 10 minutes or until softened.
Meanwhile, scoop the squash out of its skin, adding it to the pan. Pour in 5 cups water and stir in 1 tsp salt and plenty o black pepper. Bring to a boil, cover and simmer for 10 minutes.
Cool the soup slightly, then process it in a blender or food processor to a smooth puree. Alternatively,press the soup through a fine sieve with the back of a spoon. Reheat without boiling, then ladle it into warmed bowls. Top each serving with a spoonful o salsa and serve.