Begin by slicing the flank or sirloin steak into thin pieces, about 1/4 inch thick. In a mixing bowl, combine baking soda, cornstarch, kosher salt, 2 tablespoons of vegetable oil, soy sauce, and mirin. Toss the sliced beef in the marinade, cover, and refrigerate for at least 1 hour.

While the beef is marinating, whisk together the oyster sauce, hoisin sauce, 2 tablespoons of soy sauce, and brown sugar in a bowl. Set aside.

Heat 1/4 cup of vegetable oil in a large wok over medium-high heat until shimmering. Add the marinated beef in batches, stir-fry for 2-3 minutes until golden brown. Transfer to a plate.

In the same wok, lower the heat and add more oil if necessary. Add minced garlic, minced ginger, green onion whites, and dried red chilies. Sauté for 30 seconds until fragrant.

Pour the prepared sauce into the pan with the aromatics, bring to a gentle simmer. Stir occasionally, then add the browned beef and green onion tops, stirring to coat with sauce.

Continue to cook the mixture for 5 minutes over medium heat, allowing the sauce to thicken.

Serve your Mongolian Beef over a bed of white rice to soak up the sauce.