Preheat oven to 350°F.
Heat oil in a large ovenproof skillet over medium heat. Add broccoli, bell pepper and scallions; cook, stirring, until softened, 3 to 5 minutes. Combine teriyaki sauce, water, cornstarch and garlic in a measuring cup. Add to the pan along with chicken and rice. Stir to combine well.
Transfer to the oven and bake until the vegetables are tender and warmed through, about 15 minutes. Serve sprinkled with sesame seeds (or almonds), if desired.