Set your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish and set it aside.
Get the veggies ready.
If your broccoli is fresh, give it a quick steam for just a couple of minutes until tender but still bright. Frozen broccoli works too — just thaw and drain it well.
Build the base.
In a large bowl, add the cooked rice, chicken, peas, broccoli, green onions, garlic powder, onion powder, and cheese if you're using it. Gently toss everything together until it looks evenly mixed.
Make it saucy.
In a small bowl, stir together the teriyaki sauce, soy sauce, and sesame oil.
Pour this glossy mixture over the rice and chicken. Stir until everything is nicely coated. Give it a taste and add salt and pepper if needed.
Assemble the casserole.
Spoon the mixture into your prepared baking dish. Spread it out evenly and press it down just a bit so it bakes together nicely.
Bake until cozy.
Cover with foil and bake for 20 minutes. Then remove the foil and bake for another 10-15 minutes, until the top is lightly golden and everything is hot and bubbly.