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