In a large skillet with a tight-fitting lid, combine the sliced onions and balsamic vinegar. Cook over medium heat for about 20 minutes, covered. Once softened, remove the lid and continue to cook, stirring occasionally, until all the liquid has evaporated, and the onions start to caramelize. Add ⅓ cup of water, stir, and cook until the liquid evaporates again. Repeat this process until the onions are fully caramelized and soft. Set aside.

In a large mixing bowl, combine the ground turkey, finely chopped onion, Worcestershire sauce, salt, pepper, minced garlic, and dried thyme. Mix until well combined and form the mixture into 8 burger patties.

Preheat your grill to medium-high heat. Once hot, add the turkey burgers and cook for about 7 minutes on each side, flipping only once for the juiciest result. Ensure the internal temperature of the burgers reaches 165°F (75°C), or until they are cooked through with no pink remaining. In the last minute of cooking, place a slice of provolone cheese on each burger, cover the grill or skillet, and allow the cheese to melt.

Serve each turkey burger on your choice of bread, topped with a generous portion of the balsamic caramelized onions. Add any additional toppings you enjoy, such as lettuce, tomatoes, or pickles.