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
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
Heat broiler. Ladle soup into 8 ovenproof bowls; top with toast slices and cheese
Broil, 4 inches from heat, 3 to 5 min. or until cheese is melted and golden brown
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
Ingredients
2 tbsp butter
2 tbsp olive oil
3 lbs yellow onions, cut lengthwise in half, then crosswise into thin slices
