Slow Cooker French Onion Soup

 

2 lbs Vidalia onions, thinly sliced (about 8 cups)

1/4 cup butter, melted

1 1/2 tsp granulated sugar

1/12 tsp kosher salt

1/2 tsp ground black pepper

6 thyme sprigs, tied with kitchen twine

1 bay leaf

5 cups beef broth

1 Tbsp sherry vinegar

12 (1/2" thick) baguette slices

6 oz Gruyere cheese, shredded 

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 - 7 hours.  

Remove and discard thyme and bay leaf.  Stir in broth and vinegar.  Covar and cook on high until thoroughly heated, about 30 minutes.  

Preheat oven to broil, with oven rack about 6" 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.