In 4-quart Dutch oven or large saucepan, heat butter and oil over medium heat until butter is melted. Add onions; cook about 15 minutes, stirring frequently, until onions are tender and light golden brown.
Stir in brown sugar. Cook about 5 minutes, stirring occasionally, until golden brown. Add mushrooms; cook about 5 minutes, stirring occasionally, until tender. Stir in flour and salt. Stir in broth, water and sherry. Heat to boiling. Reduce heat. Cover; simmer 20 minutes.
Meanwhile, heat oven to 375°F. On ungreased cookie sheet, unroll dough into 1 large rectangle; press perforations to seal. Sprinkle Parmesan cheese and thyme evenly over dough. Starting with 1 short side, roll up. Cut roll into 8 slices; place cut sides down on cookie sheet. Press slices to form 3-inch rounds.
Bake 10 to 14 minutes or until golden brown. Remove baked rounds from oven. Sprinkle each with Swiss cheese. Bake 1 to 2 minutes longer or until cheese is melted and begins to brown.
Ladle soup into individual soup bowls. Top each with 1 crescent topper. Serve soup with additional crescent toppers.