In a 3-qt saucepan over medium heat, sauté onions in butter. Add broth, pepper, thyme and salt if desired; bring to a boil. Reduce heat; cover and simmer for 20 minutes. Add 1 2/3 cups milk. Stir flour into remaining milk until smooth; add to soup. Bring to a boil; boil and stir for 2 minutes or until thickened.
Yield: 6 Servings