Step 1

Whisk 1 Tbsp. cornstarch and ⅓ cup water in a small bowl until cornstarch is dissolved. Set slurry aside.

Step 2

Remove dark green parts from 2 scallions and thinly slice on a diagonal; set aside for serving. Thinly slice white and pale green parts of scallions and transfer to a medium bowl. Add one 1" piece ginger, peeled, finely grated, 5 garlic cloves, finely grated, ½ cup low-sodium soy sauce, ¼ cup (packed) dark brown sugar, and 1 Tbsp. apple cider vinegar and whisk to combine. Set sauce aside.

Step 3

Heat 3 Tbsp. vegetable oil in a large skillet over medium-high. Add 1½ lb. skinless, boneless chicken thighs or breasts, thinly sliced against the grain into bite-size strips, in an even layer and season with kosher salt and freshly ground pepper. Cook, undisturbed, until mostly cooked through and golden underneath, about 3 minutes. Toss and continue to cook until completely cooked through, about 1 minute more. Transfer to a plate.

Step 4

Pour reserved sauce into same skillet and reduce heat to medium. Bring to a boil, stirring constantly and scraping up browned bits. Stir slurry to combine if it has separated and add to skillet. Cook, stirring often, until sauce is thick and glossy, about 3 minutes. Return chicken to pan and toss to coat. Remove pan from heat. Top chicken teriyaki with toasted sesame seeds and reserved scallion greens. Serve with steamed white rice.