In a large bowl, whisk garlic, ginger, vinegar, brown sugar, soy sauce, orange juice, zest, and red pepper flakes until combined. Season marinade with salt and pepper to taste.
Place chicken in a sealable bag or a shallow bowl and pour marinade over top. Seal bag shut and place on a plate or cover bowl. Marinate chicken in fridge at least 2 hours or up to 12.
Preheat an outdoor grill to 400F OR prepare a large skillet to use indoors on stovetop by greasing well. Remove chicken from marinade and pat dry (do not discard marinade).
Arrange chicken thighs on grill or in skillet on stovetop and allow to cook, turning occasionally until chicken registers 165F.
Pour marinade into a small pan and bring to a simmer over medium-high heat. Reduce heat to medium-low and allow to cook 8-10 minutes, stirring frequently until marinade is reduced and slightly thickened.
Transfer chicken to a plate and drizzle with sauce. Top with green onions and sesame seeds and serve warm. Enjoy!