Melt butter in a large saucepan over medium heat. Add onions; saute until tender. Stir in flour; cook 1 minute, stirring constantly. Gradually stir in chicken broth; bring to a boil, stirring frequently, until thickened and smooth. Stir in milk, thyme, sausage and potatoes; bring just to a boil, reduce heat and simmer uncovered for 15 minutes. Stir in Parmesan cheese. Serve hot.