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In a food processor fitted with a metal blade, pulse the carrots and sweet potatoes until finely chopped.
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.
In a blender, puree the soup in batches until smooth. Return to the sauce-pan, season with salt and white pepper and stir in the remaining 1 1/2 cups of hot stock and the lemon juice. Swirl in the remaining 2 tablespoons of butter and the parsley and serve with croutons.