Slice the flank steak against the grain into approximately ⅛-inch thick slices. Sprinkle 1 teaspoon of baking soda evenly over the top side, then flip and sprinkle the remaining 1 teaspoon on the other side. Let the beef rest for 15 minutes. Rinse the baking soda off the steak with water and pat thoroughly dry. Season one side with salt and black pepper.

In a small bowl, whisk together the chicken broth and cornstarch until smooth. Add black vinegar, light brown sugar, soy sauce, dark soy sauce (if using), oyster sauce, toasted sesame oil, and red pepper flakes. Mix well.

Heat the vegetable oil in a large skillet over medium-high heat. Lay the steak slices flat in the skillet. Cook without stirring for 2-3 minutes, then stir and continue cooking for about 90 seconds until the beef is cooked through.

Lower the heat to medium. Push the beef to the edges of the skillet and add broccoli florets to the center. Stir frequently and cook the broccoli for approximately 2 minutes until slightly tender but still crisp.

Reduce the heat to low. Push the broccoli to the sides and add minced garlic to the center. Sauté until fragrant, about 1 minute.

Turn heat to medium-low and pour the prepared sauce into the skillet. Stir to combine and cook until the sauce reduces and evenly coats the beef and vegetables.

Remove from heat, stir in sliced green onions and serve immediately over steamed jasmine rice.