01 - In a medium bowl, whisk together the maple syrup, Dijon mustard, olive oil, apple cider vinegar, minced garlic, thyme, salt, and black pepper until smooth and well combined.
02 - Place the chicken breasts in a large resealable plastic bag or shallow dish. Pour the marinade over the chicken, ensuring each piece is thoroughly coated. Seal or cover and refrigerate for at least 1 hour, or up to 8 hours for deeper flavor penetration.
03 - Preheat an outdoor grill or grill pan to medium-high heat. Lightly oil the grill grates to prevent the chicken from sticking.
04 - Remove the chicken from the marinade, allowing any excess to drip off. Discard the remaining used marinade. Place the chicken on the hot grill and cook for 6 to 8 minutes per side, depending on thickness, until the internal temperature registers 165°F and the exterior is golden with distinct grill marks.
05 - Transfer the grilled chicken to a cutting board and let it rest for 5 minutes to allow the juices to redistribute. Garnish with fresh thyme sprigs if desired, then serve.