01 - In a medium bowl, whisk together the maple syrup, Dijon mustard, olive oil, apple cider vinegar, garlic, thyme, salt, and pepper until 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, up to 8 hours.
03 - Preheat grill to medium-high heat. Lightly oil the grill grates to prevent sticking.
04 - Remove the chicken from the marinade, allowing excess to drip off. Discard any remaining marinade. Grill the chicken for 6–8 minutes per side depending on thickness, or until the internal temperature reaches 165°F and the chicken is golden with distinct grill marks.
05 - Rest the grilled chicken for 5 minutes before serving. Garnish with fresh thyme if desired.