Combine 1 quart milk and rice in top of a double boiler. Place over simmering water; cover and cook 45 minutes or until rice is very soft.
Pour rice into container of an electric blender; process until smooth. Return rice to top of double boiler. Place 3 cloves in each onion half; add to rice. Cover; cook 15 minutes. Remove and discard onion. Remove boiler from heat; stir in butter, parsley, and salt. Stir in hot milk for a thinner soup, if desired. Ladle into soup bowls.