In a small bowl, whisk together the soy sauce, beef broth, and cornstarch to create the stir-fry sauce. Set aside.
Heat a large skillet or wok over medium-high heat with a tablespoon of cooking oil. Add the sliced beef flank steak and cook until browned, about 3 to 4 minutes. Remove the beef and set aside on a plate.
In the same skillet, add the sliced red, green, and yellow bell peppers along with the sliced onions. Stir-fry for 4 to 5 minutes until the vegetables are crisp-tender.
Return the cooked beef to the skillet with the vegetables. Pour the prepared soy sauce mixture over the meat and vegetables, stirring constantly for 1 to 2 minutes until the sauce thickens and coats everything evenly.
Remove from heat and serve hot over cooked white rice. Garnish with freshly chopped cilantro and sesame seeds.