For this recipe, it's important to prep your ingredients before you start cooking. If serving with rice, I recommend getting it going prior to starting the recipe. Cook rice according to package directions.
Add the sauce ingredients to a medium bowl and whisk everything together.
Cut your veggies and onion into pieces that are bite-size (make sure that the broccoli is cut quite small) so they cook at a similar rate.
Cut the chicken into 1" pieces and add them to a bowl. Sprinkle the pieces with the garlic powder and some pepper. Stir to combine. Add the cornstarch and stir until the chicken is coated.
In a deep skillet, over medium-high heat, add 2 tablespoons of the oil. Let the pan heat up for a few minutes.
Add the chicken pieces (do not crowd the pan - I recommend doing 2 batches), shaking off the excess cornstarch prior to adding them to the pan. Cook for 4 minutes and then flip and cook another 3-4 minutes or so (I flip using tongs to make it easy). Chicken should be browned and cooked through (165F). For the second batch, I add another tablespoon of olive oil to the pan. Transfer each batch of the chicken to a plate once it's done.
If the pan is dry, add a small splash of olive oil (a teaspoon or so), and then add the onions. Cook for 1 minute, stirring a few times.
Add in the broccoli and peppers, and cook for 3 minutes, stirring often. Veggies should be tender-crisp. Give it a little longer if you want them softer.
Add the chicken back to the pan and stir in the sauce (whisk it again quickly prior to pouring it in). Let it bubble until thickened (less than a minute), stirring constantly so everything is coated. Take the pan off the heat. Serve immediately over rice or noodles.