Start by cutting the chicken thighs into bite-sized pieces. I find that smaller pieces cook faster and absorb more sauce. Pat them dry lightly to help them brown better in the pan.
In a bowl, mix soy sauce, brown sugar, water, garlic, and ginger until combined. This simple mixture becomes rich and flavorful once heated. I always taste it at this stage to adjust sweetness if needed.
Heat oil in a large skillet over medium heat. Add the chicken pieces and cook until they are lightly browned on the outside. Stir occasionally so they cook evenly without sticking.
Pour the prepared teriyaki sauce into the skillet with the chicken. Let it come to a gentle simmer so the flavors meld together. The chicken finishes cooking as the sauce reduces.
Mix cornstarch with a little water to make a slurry, then pour it into the simmering sauce. Stir constantly until the sauce thickens and coats the chicken beautifully. This usually takes just a minute or two.
Once the sauce is thick and glossy, remove the pan from heat. I like to let it rest for a couple of minutes so the sauce settles and clings better to the chicken before serving.