Melt butter in heavy large pot over medium-high heat. Add onion; sauté 4 minutes. Add ginger and garlic; sauté 2 minutes. Add chopped carrots, tomatoes and lemon peel; sauté 1 minute. Add 3 cups stock and bring to boil. Reduce heat, cover partially and simmer until carrots are very tender, about 20 minutes. Cool slightly. Puree soup in batches in blender. Return soup to pot. Mix in lemon juice. Season with salt and pepper. (Can be made 1 day ahead. Cover and chill.) Bring soup to simmer, thinning with more stock, if desired. Ladle into bowls. Top each with sour cream and grated carrot.