In a skillet over medium heat, add a little oil and sauté the sliced onions until they are soft and caramelized, about 10 minutes. This step is optional, but it adds extra flavor.
Place the chicken breasts in the bottom of the crockpot. Season with salt, pepper, and dried thyme.
In a bowl, mix the beef broth, French onion soup mix, Worcestershire sauce, and minced garlic. Stir to combine.
Pour the mixture over the chicken in the crockpot. Add the caramelized onions on top.
Cover and cook on low for 6 hours or on high for 3 hours, until the chicken is fully cooked and tender.
If using, add provolone cheese on top in the last 15 minutes of cooking, replacing the lid to allow the cheese to melt.
Once done, remove the chicken and serve with a drizzle of the broth and onions. Garnish with fresh parsley if desired.