Add oil to a hot wok. Stir-fry beef until caramelized on both sides. Set aside.
In the meantime, combine all the sauce ingredients in a bowl.
Add the sesame oil to a hot wok, then add the bell pepper, ginger and garlic. Stir-fry for about 30 seconds. Add the sauce and let it simmer for 2 minutes.
Return beef to the sauce and stir to coat. Add the green onions and stir again to coat. Simmer for 1 minute and serve over steamed rice.