Begin by slicing the flank or sirloin steak into thin, even pieces. In a large bowl, toss the beef with baking soda, cornstarch, kosher salt, 2 tablespoons of vegetable oil, soy sauce, and mirin until evenly coated. Cover and refrigerate for at least 1 hour, or up to overnight.

While the beef marinates, prepare the sauce by combining 2 tablespoons of oyster sauce, 1 tablespoon of hoisin sauce, 2 tablespoons of soy sauce, and 2 tablespoons of brown sugar in a separate bowl. Whisk until smooth.

Heat a sizable wok over medium-high heat and pour in 1/4 cup of vegetable oil. Once shimmering, add the marinated beef in batches, stir-frying for about 2-3 minutes until golden.

In the same wok, lower the heat, and add minced garlic, ginger, white onion parts, and chilies. Sauté for about 30 seconds until fragrant.

Pour the prepared sauce into the wok and bring to a gentle simmer. Stir in cooked beef and green onion pieces.

Simmer for approximately 5 minutes, allowing sauce to thicken and coat the beef.

Transfer the Mongolian Beef to a platter or serve directly from the wok, garnished with green onion tops.