In a small bowl, whisk together the soy sauce, beef broth, brown sugar, sesame oil, garlic, ginger, and cornstarch. Set aside.
Heat 1 tablespoon of vegetable oil in a large wok or skillet over medium-high heat.
Add the thinly sliced beef in a single layer. Cook for 2-3 minutes, stirring occasionally, until browned. Remove the beef from the wok and set aside on a plate.
Add the remaining 1 tablespoon of vegetable oil to the wok. Add the broccoli, red bell pepper, yellow bell pepper, and yellow onion. Stir-fry for 4-5 minutes until the vegetables are crisp-tender.
Return the cooked beef to the wok. Pour the prepared sauce over the beef and vegetables.
Toss everything together and cook for an additional 1-2 minutes, or until the sauce has thickened and evenly coated the ingredients.
Garnish with sesame seeds and serve immediately over rice or noodles.