Toss the chicken with garlic powder and cornstarch in a bowl until evenly coated. This helps the chicken brown nicely and gives the sauce something to cling to.
Heat a large skillet or wok over medium-high heat and drizzle in a little oil. Once hot, add the chicken in a single layer. Cook for 5–7 minutes, flipping once, until golden and cooked through. Transfer to a plate.
In the same pan, add the onion, bell pepper, and broccoli. Stir-fry for 4–5 minutes, or until the vegetables are crisp-tender. If the pan gets too dry, add a splash of water.
Return the chicken to the pan and pour in the teriyaki sauce. Stir well to coat everything evenly.
Simmer for 2–3 minutes, or until the sauce is hot and slightly thickened.
Serve hot over rice, noodles, or as is!