In a small bowl, combine the soy sauce, brown sugar, and cornstarch, whisking until smooth, then set it aside.
Heat oil in a large skillet or wok over medium heat. Add the green and red bell pepper strips, sautéing until they turn golden, about 3 minutes. Remove them from the pan and set them aside.
Increase the heat to high, then add the thinly sliced steak to the pan. Cook the beef until browned and cooked through.
Return the sautéed bell peppers to the pan, and pour the sauce over the steak and vegetables. Stir and cook for an additional 2-4 minutes, ensuring the sauce thickens and coats everything evenly.
Serve immediately with your favorite rice or noodles. Enjoy!