Cut onions in half and then thinly slice. Place in crock pot with butter and salt. Cook on low for 8 hours.
After 8 hours, add beef broth, thyme, and balsamic vinegar to the crock pot. Cook on low for another 4 hours.
Preheat oven to 350 degrees Fahrenheit. Place soup into oven-safe bowls or dishes on a rimmed baking sheet and top with a slice of French bread and 1-2 slices of cheese.
Bake for 10-15 minutes until cheese is melted.
Enjoy!
Recipe by For the Love of Gourmet at https://fortheloveofgourmet.com/crock-pot-french-onion-soup/