In a large zip-top bag, combine the balsamic vinegar, vegetable oil, granulated sugar (or allulose), ketchup, Worcestershire sauce, Dijon mustard, kosher salt, ground black pepper, chopped green onions, and minced garlic. Seal the bag and shake it well to thoroughly mix all the ingredients into a smooth marinade.

Add the boneless, skinless chicken breast halves to the marinade in the bag. Reseal and shake the bag again to fully coat all the chicken with the marinade. Make sure each piece is evenly covered.

Place the bag with the marinated chicken in the refrigerator. Let it marinate for at least 1 hour and up to 6 hours to allow the flavors to develop and the chicken to absorb the marinade. I like to marinate it closer to 6 hours for the deepest flavor.

Take the marinated chicken out of the refrigerator. Preheat your barbecue grill or grill pan to medium heat so it's ready for cooking.

Remove the chicken breasts from the marinade and discard any leftover marinade. Place the chicken on the preheated grill and cook for about 6 to 8 minutes per side, or until the chicken is cooked through and no longer pink inside. For extra juiciness, I let the chicken rest for a few minutes off the heat before slicing.

Transfer the grilled chicken breasts to a serving platter. Serve hot, either whole or sliced, as desired.