Place onions, butter and salt in large slow cooker. Cook on low for 8-10 hours. This works best if you do it overnight.
In the morning add beef broth, Worcestershire, wine, red wine vinegar and thyme. Season with salt and pepper to taste. Cover and cook on low for another 6-8 hours.
Heat oven to 350 F. Place ovenproof bowls onto baking sheet. Ladle soup into bowls. Top each bowl with 1-2 slices of bread and about ⅓ cup of shredded cheese.
Bake for 20 minutes.
Turn on broiler and broil until golden brown, keeping a very close eye on it.
Garnish with additional thyme leaves and enjoy!