Begin by slicing the thinly sliced beef into 1/4-inch strips and cutting the onions into half-moons.

In a mixing bowl, combine the sliced beef with soy sauce, cornstarch, and a pinch of pepper. Let it rest for about 10 minutes.

Heat a large skillet over high heat and add vegetable oil.

Add half of the marinated beef to the skillet and stir-fry for about 2 minutes on each side.

Lower the heat, add the sliced onions and minced garlic to the skillet, and cook for about 4 minutes.

Return the cooked beef to the skillet, add the remaining soy sauce and cornstarch mixture, and simmer for about 1 minute.

Serve hot over steamed rice or noodles, garnished with sliced green onions and sesame seeds.