Preheat oven to 425°F (220°C). Pat thighs dry and season lightly with pepper.
Whisk honey, soy sauce, garlic, vinegar, oil, mustard, and red pepper flakes in a bowl.
Sear thighs skin-side down in an ovenproof skillet over medium-high heat until browned, about 5 minutes.
Pour most of the sauce over the thighs, reserve 2 tbsp for glazing; transfer skillet to oven and bake 18–22 minutes until internal temp is 165°F.
Warm reserved sauce in a small pan, stir in cornstarch slurry, simmer until thickened; brush over finished thighs and garnish.