Pat thighs dry; add pepper. Whisk soy sauce, mirin, sugar, ginger, garlic, water, and cornstarch until starch dissolves.
Heat oil in large skillet over medium-high. Cook unfolded thighs 6 to 7 minutes per side until browned, reducing heat if pan smokes.
Reduce to medium-low; add sauce. Turn chicken through sauce; simmer 3 to 4 minutes until glossy and thickest meat at least 165°F.
Transfer chicken to board; rest 5 minutes. Stir sauce off heat to keep smooth.
Slice thighs into thick strips; spoon warm teriyaki sauce over to serve.