Step 1: Prepare the chicken by cutting boneless, skinless chicken thighs or breasts into 1-inch bite-sized pieces. Pat them dry with paper towels.
Step 2: In a medium bowl, whisk together the soy sauce, mirin, brown sugar, minced garlic, grated fresh ginger, and rice vinegar to create the teriyaki sauce. Taste and adjust sweetness or saltiness as desired. Set aside half of the sauce for later, or use to marinate for 30 minutes.
Step 3: Heat vegetable oil in a large skillet or wok over medium-high heat. Add the chicken pieces in a single layer, ensuring not to overcrowd the pan. Cook for 3-5 minutes per side until golden brown and cooked through. Remove the chicken from the pan and set aside.
Step 4: If you want a thicker sauce, in a small bowl, whisk together the cornstarch and water to create a slurry. Set aside.
Step 5: Pour the reserved teriyaki sauce into the same skillet. Bring it to a simmer. If using, slowly whisk in the cornstarch slurry. Cook for 2-3 minutes, stirring constantly, until the sauce thickens into a glossy glaze.
Step 6: Return the cooked chicken to the skillet. Toss gently to coat all the chicken pieces evenly with the rich teriyaki glaze.
Step 7: Serve immediately over rice, garnished generously with sesame seeds and chopped green onions. Enjoy your delicious homemade teriyaki chicken recipe!