Prepare the Steak: Cut the top sirloin steaks into bite-size cubes, about 3/4-inch each.
Make the Marinade: In a small mixing bowl, whisk together the teriyaki sauce, light brown sugar, cornstarch, sesame oil, garlic powder, garlic salt, and red pepper flakes until the brown sugar and cornstarch are fully dissolved.
Marinate the Steak: Pour the marinade over the steak bites and toss until the steak pieces are evenly coated.
Cook the Steak: Drizzle a large (12-inch) skillet with olive oil and heat over high heat. Once the pan is hot, fry the steak bites in batches. Cook for 4-6 minutes total, turning as needed to cook evenly. Adjust the cooking time according to your desired level of doneness. Remove the cooked steak bites to a platter and keep warm while you cook the remaining batches. Add more olive oil between batches if needed.
Sauté the Vegetables: In the same skillet, add the sliced green pepper and onion. Sauté over medium-high heat for 2-3 minutes, or until the vegetables are crisp-tender. Adjust the seasoning with salt if needed, allowing the drippings in the pan to flavor the vegetables.
Combine and Serve: Return the cooked steak bites to the skillet, stirring to combine with the vegetables. Remove the skillet from the heat and serve immediately.