- Whisk together teriyaki sauce, brown sugar, lime juice, minced garlic, and red pepper flakes in a medium bowl until brown sugar is dissolved.
- Reserve ¼ cup of marinade for making basting sauce. Pour marinade over chicken thighs in a shallow dish. Cover and refrigerate for at least 2 hours and up to 8 hours.
- Combine reserved marinade with water and cornstarch in a small saucepan. Whisk to dissolve cornstarch. Bring mixture to a low boil over low heat. Simmer for 3-4 minutes until thickened, stirring frequently. Remove from heat and cool in a small bowl.
- Heat a large grill pan or gas grill over medium-high heat (400-425ºF) and grease with cooking oil. Remove chicken thighs from marinade and discard marinade. Place thighs on hot grill and cook for 6 minutes with lid closed if using an outdoor grill. Turn chicken and brush with reserved sauce. Cook for 6-8 minutes longer or until fully cooked.
- Serve hot with lime wedges and garnished with fresh cilantro.