01 - In a medium mixing bowl, vigorously whisk together the pure maple syrup, Dijon mustard, olive oil, apple cider vinegar, minced garlic, thyme, salt, and black pepper until fully emulsified and smooth.
02 - Arrange the chicken breasts inside a large, shallow dish or a heavy-duty resealable plastic bag. Pour the prepared marinade evenly over the poultry, ensuring every piece is thoroughly coated. Cover securely or seal the bag, then transfer to the refrigerator to marinate for a minimum of 1 hour, extending up to 8 hours for optimal flavor penetration.
03 - Set your outdoor grill or indoor grill pan to medium-high heat. Utilizing a pair of tongs and a paper towel lightly dipped in cooking oil, carefully lubricate the hot grates to prevent the meat from sticking.
04 - Retrieve the chicken from the marinade, gently shaking off any surplus liquid to avoid flare-ups; safely discard the remaining used marinade. Place the chicken directly onto the oiled grill grates. Cook for 6 to 8 minutes on each side, rotating only once, until the internal temperature of the thickest portion registers 165°F on a meat thermometer and distinct grill marks form.
05 - Transfer the fully cooked chicken from the grill to a clean cutting board or serving platter. Allow the meat to rest undisturbed for 5 minutes so the natural juices can evenly redistribute throughout the muscle fibers. Garnish with fresh thyme sprigs just before serving.