In a large saucepan combine the stock and the carrots, bring the stock to a boil, and simmer the carrots, covered, for 20 minutes, or until they are tender. In a food processor fitted with steel blades, puree the mixture in batches and force the puree through a food mill into another large saucepan. In a small skillet cook the onion in the butter over moderate heat, stirring for 3 minutes, or until it is softened. Add the onion to the saucepan with the cream, thyme, nutmeg and salt and pepper to taste. Bring soup to a boil, and simmer it for 5 minutes. Let the soup stand, covered, for 5 minutes to let the flavors blend, ladle it into a heated tureen.