I recommend prepping your ingredients before you start cooking - once the recipe is started, it goes quite fast. Be sure to cut the veggies into pieces that are bite-size (make sure that the broccoli is cut quite small) so they cook at a similar rate. 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 together.
Cut the chicken into 1" pieces and add to a bowl. Sprinkle with the garlic powder and some pepper. Stir to combine. Add the cornstarch and stir/toss until the chicken is coated.
In a skillet (use a fairly large/deep one if possible), over medium-high heat, add 2 tablespoons of the oil. Let the pan heat up for a few minutes.
Add the chicken pieces to the skillet (shake off the excess cornstarch prior to adding it to the pan). Don't crowd the pan - I recommend doing 2 batches for best browning. 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 golden and cooked through (165F). For the second batch, 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 looks 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, peppers, and carrots, and cook for 3 minutes, stirring often. Veggies will be tender-crisp. Cook for a bit 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 (about a minute), stirring constantly so everything gets coated. Take the pan off the heat. Serve immediately over rice or noodles with scallions sprinkled over top if desired.