Cut the top sirloin steaks into bite-size cubes, about 3/4-inch each.

In a small bowl, whisk together teriyaki sauce, brown sugar, cornstarch, sesame oil, garlic powder, garlic salt, and red pepper flakes until the sugar and cornstarch are dissolved.

Pour the mixture over the steak bites and toss until they are evenly coated.

Drizzle a large skillet (12-inch) with olive oil and heat over high heat.

Once the pan is hot, fry the steak bites in batches. Adjust heat to medium-high if needed to prevent burning. Cook for 4-6 minutes in total, turning to cook both sides to your desired doneness.

Remove the cooked steak to a platter and keep warm. Add more olive oil between batches as necessary.

In the same skillet, add the sliced green pepper and onion. Saute for 2-3 minutes over medium-high heat until they are crisp-tender. Adjust salt to taste, as the pan drippings will season the vegetables.

Return the steak bites to the skillet and stir to combine.

Remove the skillet from the heat and serve.