Begin by slicing flank or sirloin steak into thin strips, about 1/4 inch thick, and set aside.

In a medium bowl, combine baking soda, cornstarch, kosher salt, vegetable oil, soy sauce, and mirin. Whisk until smooth, then add sliced beef and refrigerate for at least 1 hour.

In a separate bowl, whisk together oyster sauce, hoisin sauce, soy sauce, and brown sugar. Set aside.

Heat vegetable oil in a wok or large skillet over medium-high heat. Stir-fry marinated beef in batches for 2-3 minutes until browned, then set aside.

In the same pan, add minced garlic, grated ginger, green onion whites, and dried red chilies. Sauté for 30 seconds until fragrant.

Return the cooked beef to the pan, pour in the sauce, and toss everything together. Cook for an additional 1-2 minutes.

Reduce heat and simmer Mongolian Beef for about 5 minutes to allow flavors to meld. Serve hot over rice, garnished with green onion tops.