Put the onions, butter, ¼ teaspoon of salt, and ¼ teaspoon of pepper in the slow cooker.
1
Cook on low heat for about six to eight hours, or overnight.
2
Add beef broth, garlic clove, Worcestershire sauce, balsamic vinegar, bay leaf, thyme, and remaining salt and pepper to taste.
3
Continue cooking on low for 6–8 hours.Ladle soup into broiler-safe dishes or ramekins.
4
Add cheese and French bread.Broil cheese for 3 minutes until golden and bubbling.
5