In a large skillet, combine sliced onions and balsamic vinegar over medium heat. Cover and cook for about 20 minutes, stirring occasionally.

Remove the lid and continue to cook the onions for about 10 more minutes, stirring frequently.

In a mixing bowl, combine the ground turkey, a portion of the caramelized onions, Worcestershire sauce, garlic, thyme, salt, and pepper. Mix gently and form into 8 patties.

Preheat your grill to medium-high heat, aiming for around 375°F to 400°F.

Place the patties on the grill and cook for about 7 minutes without flipping.

Add a slice of provolone cheese on each patty and close the grill lid, allowing cheese to melt for about 1-2 minutes.

Remove the burgers and let them rest for a minute. Serve on whole wheat buns topped with remaining caramelized onions.