Start by thinly slicing your flank or sirloin steak against the grain. In a bowl, mix together baking soda, cornstarch, kosher salt, vegetable oil, soy sauce, and mirin. Add the sliced beef to the marinade, ensuring it's well coated. Cover and let it marinate in the refrigerator for at least 1 hour, or up to overnight.

In a separate bowl, combine oyster sauce, hoisin sauce, soy sauce, and brown sugar. Whisk the ingredients together until smooth and well blended.

Heat a large wok or non-stick pan over medium-high heat and add vegetable oil. Once shimmering, add the marinated beef in batches. Stir-fry for 2-3 minutes until caramelized and cooked through. Set aside.

In the same wok, reduce heat slightly and add minced garlic, ginger, green onion whites, and dried red chilies. Sauté for about 30 seconds until fragrant.

Pour the prepared sauce mixture into the wok, stirring well. Bring to a simmer, then add the cooked beef, tossing to coat in the sauce.

Let the beef and sauce simmer for about 5 minutes, stirring occasionally to meld the flavors. Adjust heat to maintain a gentle simmer.

Serve the hot Mongolian Beef over fluffy rice or steamed vegetables, garnished with green onion tops.