In a small saucepan combine 1/2 cup soy sauce, 1/4 cup honey, 1/2 cup of the cold water, 3/4 teaspoon ground ginger, and 1/2 teaspoon garlic powder. Bring the mixture to a boil over medium-high heat.
While the sauce is boiling, whisk together the remaining 1/4 cup cold water and 2 tablespoons cornstarch in a small bowl until smooth.
Gradually pour the cornstarch mixture into the boiling sauce while stirring constantly. Reduce heat to a simmer and continue stirring until the sauce thickens, about 1–2 minutes. Remove from heat and set aside. (If you want a sweeter sauce, stir in the additional 1/4 cup honey now.)
Preheat the oven to 400°F (200°C). Spray a 9x13-inch casserole dish with nonstick spray and set aside.
In a large skillet, heat 1 tablespoon sesame oil over medium heat until shimmering. Add the 1 pound cubed chicken in a single layer and cook until the pieces are golden on the outside, about 4–6 minutes. (They do not need to be fully cooked through yet.)
Add 1/2 cup diced yellow onion, 3 minced garlic cloves, and 2 teaspoons fresh minced ginger to the skillet. Sauté, stirring occasionally, until the onions soften, about 5 minutes.
Add 3 cups cooked brown rice, 1 cup small broccoli florets, 1/2 cup shredded carrots, and 1 cup snow peas (halved) to the skillet. Pour the prepared teriyaki sauce over everything and stir until all ingredients are evenly coated.
Transfer the mixture to the prepared 9x13-inch baking dish and spread into an even layer.
Bake for 30 minutes, or until the chicken is cooked through and the vegetables are tender but not mushy. Remove from the oven and serve immediately.