1In 4-quart Dutch oven, melt butter over medium heat. Cook onions in butter 35 minutes, stirring occasionally, until lightly browned and very tender. Add garlic; cook 1 minute. Add 1 cup of the broth, stirring to loosen browned bits from bottom of pan. Stir in 2 cups broth. Heat to boiling; reduce heat. Cover; simmer 10 minutes. Remove from heat; cool 30 minutes.
2Meanwhile, in 12-inch skillet, heat oil over medium-high heat. Cook mushrooms in oil 8 to 10 minutes or until lightly browned and liquid has evaporated; stir in 1/2 teaspoon of the salt. Remove from heat; keep warm.
3In blender, place half of the onion-broth mixture. Cover; blend on medium speed 2 minutes or until smooth. Pour into large bowl. Repeat with remaining mixture. Return pureed mixture to Dutch oven. Stir in remaining 1 cup broth and the whipping cream; heat to boiling. Remove from heat; stir in sherry, lemon juice, pepper and remaining 1/4 teaspoon salt. Top with mushrooms.