There aren't many things a warm batch of classic French onion soup can fix, and this slow-cooker version saves you from standing by the stovetop for hours.
2 lbs. Vidalia onions, thinly sliced (about 8 cups sliced)
1/4 cup unsalted butter, melted
1 1/2 tsp. granulated sugar
1 1/2 tsp. kosher salt
1/2 tsp. black pepper, plus more for garnish
6 thyme sprigs, tied with kitchen twine
1 dried bay leaf
5 cups lower-sodium beef broth
1 Tbsp. sherry vinegar
12 (1/2-in.-thick) baguette slices
6 oz. Gruyère cheese, shredded (about 1 1/2 cups)
Fresh thyme leaves
Stir together onions, melted butter, sugar, salt, and pepper in a 6-quart slow cooker until onions are fully coated. Place thyme and bay leaf on onion mixture. Cover and cook on HIGH, stirring once halfway through, until onions are deep golden brown and caramelized, 6 to 7 hours.
Remove and discard thyme and bay leaf. Stir in broth and vinegar. Cover and cook on HIGH until thoroughly heated, about 30 minutes.
Preheat oven to broil, with oven rack about 6 inches from heat. Spoon soup into 6 ovenproof ramekins; place on a large rimmed baking sheet. Top each ramekin with 2 baguette slices; divide cheese evenly among ramekins (about 1/4 cup each).
Broil until cheese is melted and golden brown, about 3 minutes. Sprinkle with fresh thyme leaves and black pepper. Serve immediately.