Heat the olive oil in a large saucepan over medium heat. Saute the onion until it is translucent and just barely golden. Add the pumpkin and sweet potatoes, and stir, mashing gently, until heated through. Add the vegetable broth and heat to boiling, then reduce to a simmer. Let simmer for 10-15 minutes, then remove from heat. Puree the soup with an immersion blender (ideally) or in a food processor, adding more broth if needed to achieve a proper soup consistency. Add salt to taste.