In a large mixing bowl, combine the cooked chicken, cooked rice, teriyaki sauce, frozen mixed vegetables, and garlic powder. Stir well until all ingredients are evenly coated.
Season the mixture with salt and pepper to taste.
Transfer the mixture to a greased 9x13 inch baking dish and spread it evenly. Top with shredded cheese and chopped green onions.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly, and the casserole is heated through.
Remove from the oven and let it cool for a few minutes before serving.