01 - In a medium mixing bowl, whisk together the pure maple syrup, Dijon mustard, olive oil, apple cider vinegar, minced garlic, thyme leaves, kosher salt, and black pepper until thoroughly combined and smooth.
02 - Place the chicken breasts in a large resealable plastic bag or a shallow glass dish. Pour the marinade over the chicken, ensuring each piece is evenly coated on all sides. Seal the bag or cover the dish tightly and refrigerate for a minimum of 1 hour, or up to 8 hours for deeper flavor penetration.
03 - Preheat an outdoor grill or grill pan to medium-high heat (approximately 375°F to 400°F). Lightly oil the grill grates using a folded paper towel dipped in vegetable oil to prevent the chicken from sticking.
04 - Remove the chicken breasts from the marinade, allowing any excess to drip off naturally. Discard all remaining used marinade. Place the chicken onto the preheated grill and cook for 6 to 8 minutes per side, depending on thickness, until the internal temperature registers 165°F on a meat thermometer and the exterior displays golden grill marks.
05 - Transfer the grilled chicken to a clean cutting board and allow it to rest for 5 minutes so the juices redistribute evenly throughout the meat. Slice and serve, garnished with fresh thyme sprigs if desired.