Melt 2 tablespoons of the butter in a medium saucepan. Add the onion, cover and cook over moderately high heat, stirring occasionally, until softened. Add the carrots and sweet potatoes, cover and cook, stirring once, until the vegetables are just beginning to soften, about 2 minutes. Add the tomato paste and 5 cups of the hot stock and bring to a boil. Cover partially and cook over moderate heat until the vegetables are tender, about 15 minutes.