Cut the steak in strips that are about 1/4 inch thick and about 3 inches long.
In a medium to large skillet, heat 2 Tablespoons of olive oil over medium-high heat. Add the meat and season with salt and pepper. Add 1/4 cup of teriyaki sauce and brown the meat on all sides, about 5 minutes. You do not want to cook the meat completely through at the time.
Remove the meat to a plate and tent with foil.
In the same skillet, add 2 Tablespoons olive oil. Add the bell peppers and onions and season with salt and pepper. Cook for about 3 minutes. You want them to keep their firmness. Add the garlic and cook for another 3 minutes.
Return the meat to the skillet. Add the beef stock and remaining 1/4 cup of teriyaki sauce. Cook for another 5 minutes.
Mix the cornstarch and water in a small bowl or cup. Drizzle the mixture around the skillet and mix well. This will thicken the sauce a bit. Cook for about 3-4 minutes.
Garnish with green onions and sesame seeds.
Top Round Steak - you can use flank steak or sirloin for this meal.
Teriyaki Sauce - many people like to make their own Teriyaki Sauce, however, Kikkoman's Teriyaki sauce is just as good. So I buy that. But feel free to use your own if you'd like.
Toasted Sesame Seeds
Place a small skillet on low heat. Add the sesame seed to the skillet and stir often. When the seed begin to glisten and look wet, remove them from the heat and transfer them to a small bowl.
Colorful Bell Peppers. I like my meals to be colorful, so I like to use a variety of colors when it comes to my bell peppers.
Cook in Batches. Often times the beef will give off some water. If you overcrowd the pan, too much water will come out and you will end up steaming the meat instead of searing it. To avoid this, either use a large skillet, like 14", or cook the meat in batches.