In a small bowl, whisk the balsamic vinegar, olive oil, garlic, and Italian seasoning. In a large sealable plastic bag, combine the turkey breasts and the marinade. Press as much air as you can from the bag, seal it, and toss a few times to coat the turkey breasts in the marinade. Refrigerate for at least 30 minutes and up to 8 hours.

Remove the turkey breasts from the marinade and lay on a cutting board. Slice into each breast lengthwise, taking care not to cut all the way through, and open it like a book. Layer one side of each turkey breast with spinach leaves, roasted red peppers, and a slice of provolone cheese.

Preheat your gas grill over medium-high flames. Grill the turkey breasts, covered, until well-browned on both sides and they reach an internal temperature of 165 degrees F, 6-8 minutes per side. Serve.