Preheat oven to 400 degrees F (200 degrees C). Lightly grease a 9x13-inch baking dish or line it with foil for easier cleanup.

Whisk together the brown sugar, balsamic vinegar, soy sauce, Worcestershire sauce, Dijon mustard, garlic powder, onion powder, crushed red pepper flakes, salt, and pepper in a medium bowl until sugar is mostly dissolved.

Place the chicken thighs in the prepared baking dish skin side up in a single layer.

Pour the sauce evenly over the chicken and use a spoon to coat each piece well, including under the edges of the skin.

Bake uncovered for 40 to 45 minutes, or until the internal temperature reaches 165 degrees F (75 degrees C) and the sauce has reduced into a thick, sticky glaze.

Spoon the pan sauce back over the chicken at least twice during baking to build up the glaze. Do this at the 20 and 35 minute marks.

Let rest for 5 minutes before serving. Garnish with chopped parsley or green onions if desired.