Slice the chicken breasts into 1-inch strips for even cooking. In a shallow bowl, combine the garlic powder, black pepper, and 4 tablespoons cornstarch—this coating will create a golden, crispy crust on the chicken. Toss the chicken strips in this mixture until evenly coated, shaking off any excess. While the chicken rests, dice the onion into bite-sized pieces, cut the broccoli into small florets, and slice the red bell pepper into 1-inch pieces. Having everything prepped and ready will keep the cooking process smooth and fast.

In a small bowl, whisk together the chicken broth, mirin, soy sauce, rice vinegar, and brown sugar until the sugar begins to dissolve. Add the minced garlic, grated ginger, 1.5 tablespoons cornstarch, and sesame oil, stirring until the cornstarch is fully incorporated and there are no lumps. This sauce will thicken beautifully once it hits the hot pan, so I like to taste it now and adjust the sweetness or saltiness to your preference before cooking begins—it's easier than fixing it later.

Heat 2 tablespoons of olive oil in a large skillet over medium-high heat until it shimmers—this high heat is essential for achieving that crispy golden crust. Working in batches to avoid overcrowding, add half the seasoned chicken strips and cook undisturbed for 4 minutes until golden brown on the first side. Flip and cook for another 4 minutes until the second side is equally golden and the chicken is cooked through. Transfer the cooked chicken to a clean plate and repeat with the remaining 2 tablespoons of oil and the second batch of chicken.

In the same skillet (no need to clean it—those browned bits add flavor), add the diced onion and cook for 1 minute, stirring occasionally until it becomes fragrant and slightly softened. Add the broccoli florets and red bell pepper slices, tossing everything together, and cook for 3 minutes, stirring frequently, until the vegetables are tender-crisp and beginning to caramelize at the edges. This quick cooking preserves the vegetables' fresh texture and vibrant color.

Return the cooked chicken from Step 3 to the skillet, nestling it among the vegetables. Pour in the teriyaki sauce mixture from Step 2, stirring gently to combine all components. Increase the heat to medium-high and cook, stirring constantly, for 1-2 minutes until the sauce bubbles and thickens into a glossy glaze that coats the chicken and vegetables. The cornstarch in the sauce will continue to thicken as it cooks, so don't walk away—constant stirring prevents lumps and ensures even cooking.

Remove the skillet from heat and transfer the teriyaki chicken stir-fry to serving plates or a serving bowl. Scatter the sliced green onions over the top as a fresh garnish—they add a bright onion flavor and visual pop that complements the rich sauce. Serve immediately while everything is hot and the sauce is still glossy.