Make the Sauce: In a small bowl, whisk together soy sauce, mirin, brown sugar, grated ginger, and minced garlic. In a separate small bowl, mix the cornstarch and water to make a slurry. Set aside.

Cook the Chicken: Heat 1 tbsp of oil in a large skillet or wok over medium-high heat. Add the chicken in a single layer and cook for 5-7 minutes, until golden and cooked through. Remove from the skillet and set aside.

Cook Vegetables: Add the remaining 1 tbsp of oil to the skillet. Add the sliced onion and carrots, cooking for 3-4 minutes until they start to soften. Add the broccoli florets and sliced bell pepper. Cook for another 4-5 minutes until vegetables are crisp-tender.

Combine: Return the cooked chicken to the skillet with the vegetables. Pour the teriyaki sauce mixture over top and bring to a simmer.

Thicken and Serve: Stir the cornstarch slurry and pour it into the simmering sauce. Stir constantly for 1-2 minutes until the sauce thickens and becomes glossy. Serve immediately over rice, garnished with sesame seeds and green onions.