01 - In a medium mixing bowl, whisk together the maple syrup, Dijon mustard, olive oil, apple cider vinegar, minced garlic, thyme, salt, and 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 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 to prevent sticking.
04 - Remove the chicken from the marinade, allowing any excess to drip off. Discard the remaining marinade. Place the chicken on the 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 develops golden grill marks.
05 - Transfer the grilled chicken to a cutting board and allow it to rest for 5 minutes to redistribute juices. Slice and garnish with fresh thyme sprigs if desired. Serve immediately.