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.
After 6-8 hours, preheat oven to 350 degrees F. Place oven proof bowls onto baking sheet.
Ladle soup into oven proof bowls.
Top with 1-2 slices of bread and top each bowl with ~? cup of shredded cheese.
Bake in preheated oven for 20 minutes.
Turn on broiler and broil until golden brown, keeping a very close eye on it.
Garnish with additional thyme leaves.
Enjoy!