Combine all ingredients except 1/2 tablespoon lemon juice in a 3-quart heavy saucepan and bring to a boil over moderate heat, stirring occasionally. Reduce heat and simmer, covered, until carrots are tender, about 30 to 40 minutes.
Using a blender or food processor, purée the soup in batches, being careful with hot liquid. Place puréed soup in the refrigerator to completely cool down.
To serve soup, stir in remaining 1/2 tablespoon lemon juice and season to taste with salt and pepper. Garnish with a drizzle of honey, sour cream and a sprig of fresh thyme.