Slice beef thinly against the grain and toss with 1 tbsp cornstarch and 1 tbsp soy sauce. Let sit for 10 minutes.
Slice all bell peppers and the white onion into even strips. Mince garlic and ginger.
Whisk together remaining soy sauce, beef broth, and remaining 1 tbsp cornstarch to make the sauce.
Heat 1 tbsp vegetable oil in a wok or large skillet over high heat. Sear the beef until browned, about 2-3 minutes. Remove beef and set aside.
Add remaining 1 tbsp oil to the pan. Stir-fry bell peppers, onion, garlic, and ginger until vegetables are tender-crisp, about 4-5 minutes.
Return beef to the pan. Pour the sauce over the mixture and stir until thickened, about 1-2 minutes.
Garnish with sliced green onion and sesame seeds. Serve hot over cooked white rice.