In 5-quart Dutch oven, melt butter over medium heat. Cook onions in butter 15 to 20 minutes until very soft and beginning to brown; stir in brown sugar and vinegar. Continue cooking 5 to 10 minutes longer until onions are golden brown. Stir in remaining ingredients except sour cream and fresh sage. Heat to boiling; reduce heat. Cover and simmer 5 minutes. Remove from heat. With immersion blender*, puree soup until smooth. Ladle into bowls; garnish with sour cream and fresh sage.
