Melt 1 tablespoon butter in a large Dutch oven over medium heat. Add onion to pan; cook 4 minutes or until tender, stirring occasionally. Stir in cinnamon and nutmeg; cook 1 minute, stirring constantly. Move onion mixture to side of pan; add remaining 2 tablespoons butter to open space in pan. Increase heat to medium-high; cook 1 minute or until butter begins to brown. Add sweet potatoes, 3 1/2 cups water, broth, and carrot; bring to a boil. Cover, reduce heat, and simmer 35 minutes or until vegetables are tender.
Place half of soup mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Pour into a large bowl. Repeat procedure with remaining soup mixture. Stir in half-and-half, salt, and pepper. Ladle about 1 cup soup into each of 8 bowls; top each serving with about 2 teaspoons sour cream and 3/4 teaspoon parsley.