01 - In a medium bowl, whisk together the maple syrup, Dijon mustard, olive oil, apple cider vinegar, garlic, thyme, salt, and pepper until the mixture is fully emulsified.
02 - Place the chicken breasts in a large resealable plastic bag or a shallow dish. Pour the marinade over the chicken, ensuring each piece is evenly coated. Seal the bag or cover the dish and refrigerate for a minimum of 1 hour, up to 8 hours.
03 - Preheat your grill to medium-high heat. Lightly oil the grill grates to prevent the chicken from sticking during cooking.
04 - Remove the chicken from the marinade, allowing the excess liquid to drip off. Discard the remaining used marinade.
05 - Grill the chicken for 6 to 8 minutes per side, depending on thickness, until the internal temperature reaches 165°F and the meat displays golden grill marks.
06 - Transfer the grilled chicken to a plate and let it rest for 5 minutes before serving to retain juices. Garnish with additional fresh thyme if desired.