Slice the flank steak against the grain into thin strips, about 1/4 inch thick. In a mixing bowl, combine the beef with soy sauce, peanut oil, cornstarch, and optional baking soda. Let the beef marinate for 10 minutes.

In a medium bowl, whisk together all the sauce ingredients, ensuring the brown sugar is fully dissolved.

Place a large skillet over medium-high heat and add 1/4 cup of water. Once boiling, add the broccoli florets, cover, and steam for 1 minute until bright green.

Heat a tablespoon of peanut oil in the same skillet until shimmering. Spread the marinated beef in a single layer and sear for 30 seconds, then flip and cook for an additional 1-2 minutes.

Add minced garlic and ginger to the skillet, stirring quickly for about 30 seconds.

Return the steamed broccoli to the skillet with the beef and pour in the prepared sauce. Stir together and cook for another minute until the sauce thickens.

Serve hot over rice or alongside noodles.