Begin by slicing the flank or sirloin steak into thin strips, about ¼ inch thick. This ensures quick cooking and tenderness. Once sliced, set the beef aside on a plate, allowing it to come to room temperature, which typically takes about 15-20 minutes, to ensure even marination.

In a mixing bowl, whisk together baking soda, cornstarch, kosher salt, 2 tablespoons of vegetable oil, 2 teaspoons of soy sauce, and 1 teaspoon of mirin. Add the sliced beef, ensuring every piece is thoroughly coated in the marinade. Cover with plastic wrap and refrigerate for at least 1 hour, or overnight if possible, for maximum tenderness and flavor infusion.

While the beef is marinating, prepare the sauce by combining 2 tablespoons of oyster sauce, 1 tablespoon of hoisin sauce, and another 2 tablespoons of soy sauce in a separate bowl. Add 2 tablespoons of brown sugar and mix well until the sugar dissolves completely, creating a rich, velvety sauce. Set the sauce aside for later.

Heat ¼ cup of vegetable oil in a wok or large skillet over medium-high heat until it shimmers. Add the marinated beef in batches, cooking for about 2-3 minutes per batch until golden brown. Once browned, transfer the beef to a plate and allow it to rest; this step ensures all pieces are cooked evenly without steaming.

In the same pan, add a bit more oil if necessary, then sauté 5-6 cloves of minced garlic, 2 teaspoons of minced ginger, and the whites of the green onions, along with 8 dried red chilies. Stir-fry for about 30 seconds until you can smell the fragrant aroma, being careful not to burn the garlic.

Pour the prepared sauce into the wok and bring it to a simmer. Allow it to bubble for 1-2 minutes, letting the flavors meld. Then, add the browned beef back into the pan along with the green onion greens. Toss everything together, ensuring the beef is well-coated in the savory sauce.

Reduce the heat to medium-low and let the Mongolian Beef simmer for about 5 minutes. This will help thicken the sauce and allow the meat to absorb the rich flavors. You'll know it’s ready when the sauce becomes glossy and coats the beef beautifully.

Finally, serve your mouthwatering Mongolian Beef over warm white rice or cauliflower rice for a low-carb option. The dish is best enjoyed fresh, allowing the tender beef and rich sauce to enhance every bite. Add a few extra green onions on top for a delightful crunch and color.