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

In a 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 vegetable oil in a large skillet over medium-high heat. Lay the steak slices flat and cook without stirring for 2-3 minutes, then stir and cook for another 90 seconds until cooked through.

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

Reduce heat to low. Push the broccoli to the sides and add minced garlic to the center, sautéing 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 thickens slightly.

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