Preheat your oven to 400°F (200°C).
In a mixing bowl, combine the brown sugar, Dijon mustard, olive oil, minced garlic, salt, black pepper, thyme, and apple cider vinegar if using. Whisk the ingredients until well mixed.
Place the chicken thighs in a baking dish or oven-safe skillet. Pour the marinade mixture over the chicken, making sure to coat each piece evenly. For deeper flavor, marinate the chicken in the refrigerator for up to 2 hours before baking.
Place the baking dish in the preheated oven.
Bake for 25-30 minutes, or until the chicken reaches an internal temperature of 165°F (75°C) and the skin turns golden brown.
Remove the chicken from the oven and let it rest for 5 minutes to allow the juices to redistribute.
For extra crispy skin, broil the chicken for an additional 2-3 minutes at the end of the baking time.
