In a bowl, combine the sliced flank steak, soy sauce, sesame oil, and cornstarch. Let it marinate for about 15 minutes.
Heat a large skillet or wok over medium-high heat and add vegetable oil.
Add the marinated beef to the pan and stir-fry until browned. Remove beef and set aside.
In the same pan, add more oil if necessary, and sauté the chopped onion and garlic until fragrant.
Add the cooked rice to the pan and stir-fry, breaking apart any clumps.
Push the rice to one side of the pan, pour the beaten eggs into the empty side, and scramble them.
Add the cooked beef back to the pan, along with green onions, and mix everything well.
Season with salt and pepper to taste. Serve hot.