In a mixing bowl, whisk together the maple syrup, Dijon mustard, olive oil, minced garlic, salt, pepper, thyme, and apple cider vinegar until well combined.
Place the chicken breasts in a resealable plastic bag or shallow dish and pour the marinade over the chicken, ensuring each piece is well coated. Seal the bag or cover the dish and refrigerate for at least 30 minutes (up to 2 hours for more flavor).
Preheat the oven to 400°F (200°C).
Remove the chicken from the marinade, reserving the marinade for later use. Place the chicken breasts in a baking dish.
Bake the chicken breasts in the preheated oven for 20-25 minutes, or until the chicken is cooked through (internal temperature of 165°F or 75°C).
Meanwhile, in a small saucepan, bring the reserved marinade to a gentle boil and let it simmer for about 3-5 minutes until slightly thickened and bubbly.
Once the chicken is cooked, drizzle the thickened marinade over the chicken breasts and return them to the oven for an additional 5 minutes to caramelize the top.
Remove the chicken from the oven and let it rest for a few minutes before slicing.
Serve hot, garnished with sliced green onions if desired.
Notes
For best flavor, marinate the chicken for up to 2 hours.