Slice the flank steak against the grain into approximately ⅛-inch thick slices. Sprinkle 1 teaspoon of baking soda evenly over both sides of the steak. Let the beef rest for 15 minutes, then rinse off the baking soda and pat dry. Season one side with salt and black pepper.

In a small bowl, whisk together the chicken broth and cornstarch until smooth. Add the 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 and cook for 2-3 minutes without stirring, then stir and cook for an additional 90 seconds until cooked through.

Lower the heat to medium and push the beef to the edges of the skillet. Add the broccoli florets to the center and stir frequently for about 2 minutes until slightly tender but still crisp.

Reduce the heat to low. Push the broccoli to the sides and add the minced garlic to the center with a small amount of oil. Sauté until fragrant, about 1 minute.

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

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