In a large mixing bowl, combine baking soda, cornstarch, kosher salt, vegetable oil, soy sauce, and mirin. Add the thinly sliced flank or sirloin steak, ensuring every piece is well-coated with the marinade. Cover the bowl with plastic wrap and refrigerate for at least 1 hour.

While the beef marinates, prepare the sauce by whisking together oyster sauce, hoisin sauce, soy sauce, and brown sugar in a separate bowl. Set aside.

In a wok or large non-stick pan, heat enough vegetable oil over medium-high heat until it shimmers.

Carefully add marinated beef to the hot oil in batches, stir-fry for 2-3 minutes until browned, then transfer to a plate.

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

Pour in the prepared sauce and bring to a simmer. Add the cooked beef and green onion tops, stir to coat.

Allow to simmer for an additional 5 minutes until the sauce thickens slightly.