If desired, heat a large frying pan over medium-high heat and add the olive oil. Once hot, add the chicken breasts and sear for 2-3 minutes on each side until browned.
Transfer the chicken breasts to the crockpot.
In the same frying pan, add the sliced onions and cook over medium heat for about 10 minutes, stirring frequently until they are soft and golden brown. Add the minced garlic toward the end and cook for an additional minute.
Pour the beef broth and Worcestershire sauce into the crockpot over the chicken. Sprinkle the thyme, salt, and black pepper.
Add the sautéed onions and garlic on top of the chicken in the crockpot.
Cover and cook on low for 6 hours.
Once done, shred the chicken with two forks while it's in the crockpot or remove it, shred it, and return it to the pot.
Sprinkle the shredded provolone cheese over the chicken and cover for an additional 10-15 minutes, until the cheese is melted.
Serve warm, making sure to spoon some of the onion and broth mixture over the chicken.