1/4 cup unsalted butter
6 thyme sprigs
1 bay leaf
5 pounds large sweet onions, vertically sliced (about 16 cups)
1 tablespoon sugar
6 cups unsalted beef stock (such as Swanson)
2 tablespoons red wine vinegar
1?1/2 teaspoons kosher salt
1 teaspoon black pepper
24 (1/2-ounce) slices whole-grain French bread baguette
5 ounces Gruyère cheese, shredded (about 1 1/4 cups)
Place butter, thyme, and bay leaf in the bottom of a 6-quart electric slow cooker. Add onions; sprinkle with sugar. Cover and cook on HIGH for 8 hours.
Remove thyme and bay leaf; discard. Add stock, vinegar, salt, and pepper; cook, covered, on HIGH for 30 minutes.
Preheat broiler to high.
Arrange bread in a single layer on 2 baking sheets; broil 30 seconds on each side or until toasted. Place 1 cup soup in each of 12 (8-ounce) ramekins or ovenproof bowls, or follow freezing instructions. Top each serving with 2 bread slices and about 2 tablespoons cheese. Place 6 ramekins on a jelly-roll pan; broil 2 minutes or until cheese melts and begins to brown. Repeat procedure with remaining 6 ramekins, bread slices, and cheese.