#1 Melt butter with oil in Dutch oven on medium-high heat. Add onions cook 10 min. or until tender, stirring frequently. Simmer on medium-low heat 1 hour or until onions are golden brown, stirring occasionally and adding garlic and thyme for the last 5 min #2 Stir in sherry cook and stir on medium-high heat 1 min. Add beef broth, Worcestershire sauce, vinegar stir. Bring to boil, stirring occasionally. Cover, simmer on medium-low heat 20 min., stirring occasionally. Remove and discard thyme sprigs #3 Heat broiler. Ladle soup into 8 ovenproof bowls top with toast slices and cheese #4 Broil, 4 inches from heat, 3 to 5 min. or until cheese is melted and golden brown #5 Optional: to make ahead, prepare soup as directed cool, then refrigerate up to 2 days. When ready to serve, bring soup to boil. Ladle into ovenproof bowls and continue as directed.