Make the Teriyaki Sauce:
In a medium saucepan, combine soy sauce, 1/2 cup water, brown sugar, ground ginger, and minced garlic. Stir until sugar begins to dissolve.
Place over medium heat and bring to a gentle simmer until sugar dissolves completely.
In a small bowl, mix cornstarch with 2 tablespoons cold water until smooth with no lumps.
Pour cornstarch slurry into simmering sauce while stirring constantly. Cook for 2 minutes until thickened. Remove from heat and set aside.
Prepare the Chicken:
Cut chicken breasts into bite-sized pieces, roughly 1-inch cubes.
Heat a large skillet or wok over medium-high heat. Add chicken in a single layer.
Cook for 5-7 minutes, stirring occasionally, until cooked through and lightly golden (internal temperature 165°F).
Assemble the Casserole:
Reduce heat to medium-low. Add steamed stir-fry vegetables to the skillet with chicken. Toss together for 2 minutes.
Pour prepared teriyaki sauce over chicken and vegetables. Stir gently to coat everything evenly.
Add cooked rice and fold in carefully until evenly distributed.
Continue cooking for 2-3 minutes, stirring occasionally, until everything is heated through.
Serve hot, garnished with sesame seeds and sliced green onions if desired.