Preheat oven to 350°F.
Heat 2 tablespoons sesame oil in a large ovenproof skillet over medium heat. Add 3 cups broccoli, 1 cup bell pepper and 1 cup scallions; cook, stirring, until softened, 3 to 5 minutes.
Combine ½ cup teriyaki sauce, ½ cup water, 1½ tablespoons cornstarch and the grated garlic in a measuring cup. Add to the pan along with 3 cups chicken and 3 cups rice. Stir to combine well.
Transfer to the oven and bake until the vegetables are tender and the casserole is warmed through, about 15 minutes.
Serve sprinkled with sesame seeds (or almonds), if desired.
