In a saucepan cook the onion and garlic in the butter, covered, over moderately low heat, stirring occasionaly, until it is softened; add the carrots and broth and simmer the mixture, covered, for 20 minutes. Reserving 8 carrot slices for garnish, in a blender puree the mixture in batches until it is smooth and transfer the puree to the saucepan. Add the gingerroot and the thyme; simmer the soup, stirring, for 10 minutes, and strain it through a fine sieve into a bowl pressing hard on the solids. Ladle the soup into heated small bowls and garnish it with the reserved carrot slices. Makes about 4 cups serving 4 to 8.