Slice the beef: Freeze the flank steak for 20 minutes to firm it up, then slice it about 1/4 inch thick across the grain. Look for the lines running through the meat and cut perpendicular to them.

Velvet the beef: Toss the sliced beef with 2 tablespoons of the soy sauce, 1 tablespoon of the cornstarch, and the baking soda until every piece is coated. Let it sit on the counter for 15 minutes while you prep everything else.

Whisk the sauce: In a small bowl, whisk together the beef broth, remaining 1/4 cup soy sauce, oyster sauce, brown sugar, sesame oil, garlic, ginger, and remaining 1 tablespoon cornstarch until the sugar dissolves and no lumps remain.

Cook the broccoli: Heat 1 tablespoon of the vegetable oil in a large skillet or wok over medium-high heat. Add the broccoli and cook for 2 minutes, then add the water, cover, and steam for 3 minutes until bright green and crisp-tender. Transfer to a plate.

Sear the beef: Turn the heat to high and add the remaining 1 tablespoon vegetable oil. Spread the beef in a single layer and sear for about 2 minutes per side, just until browned with no pink remaining in the center. USDA recommends 145°F for beef, and strips this thin get there fast, so don't walk away.

Thicken the sauce: Give the sauce a quick re-whisk (the cornstarch settles) and pour it into the pan. Simmer for 2-3 minutes, stirring, until it darkens and thickens enough to coat the back of a spoon.

Bring the beef and broccoli together: Return the broccoli to the pan and toss for 1 minute so every floret and every strip of beef picks up the sauce.

Serve: Spoon over hot cooked white rice and finish with a sprinkle of sesame seeds.