Start by thinly slicing your beef against the grain. Then, in a medium bowl, whisk together the soy sauce, cornstarch, brown sugar, and rice vinegar. Add the sliced beef to the bowl and toss to coat it evenly in the marinade.

While not strictly necessary, allowing the beef to marinate for even 15-20 minutes can enhance the flavor. You can cover the bowl and let it sit at room temperature or in the refrigerator.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the minced garlic, minced ginger, and red pepper flakes (if using). Cook for about 30-60 seconds, stirring constantly, until fragrant.

Add the remaining tablespoon of vegetable oil to the skillet. Then, add the marinated beef in a single layer. It is very important not to overcrow the pan. If you are using a smaller skillet, consider doing it in batches. Cook for 2-3 minutes per side, until browned and cooked through. The beef should be slightly crispy on the edges.

Once the beef is cooked, drizzle the sesame oil over the beef and stir to combine.

In a small, dry skillet, toast the sesame seeds over medium heat for 2-3 minutes, shaking the pan frequently, until they are lightly golden and fragrant. Watch them closely, as they can burn quickly.

Remove the beef for serving and make sure to sprinkle it with sesame seeds.

Transfer the beef to a serving dish and garnish with the toasted sesame seeds and sliced green onions. Serve immediately over rice or noodles.