Heat the olive oil in a skillet over medium heat. Add the sliced onions and sauté until they are caramelized, about 10 minutes. Stir occasionally to ensure even cooking.
In the slow cooker, place the chicken breasts at the bottom. Season them with garlic powder, black pepper, and dried thyme.
Add the caramelized onions on top of the chicken breasts.
Pour in the beef broth and Worcestershire sauce, ensuring the chicken is mostly submerged in the liquid.
Cover the slow cooker and cook on low heat for 6 hours, or until the chicken is tender and cooked through.
About 15 minutes before serving, sprinkle the shredded provolone cheese over the chicken and cover again to let it melt.
Once done, serve the chicken hot, garnished with fresh parsley if desired. You can also spoon some of the delicious broth over the chicken.