Slice the flank steak against the grain into 1/4-inch strips. Toss in a bowl with 1 tbsp cornstarch, 1 tbsp soy sauce, and baking soda. Let sit for 15 minutes.

In a small bowl, whisk together the remaining soy sauce, dark soy sauce, oyster sauce, brown sugar, beef broth, and the remaining 1 tbsp of cornstarch until smooth.

Bring a small pot of water to a boil. Blanch the broccoli florets for 60 seconds until bright green. Drain and rinse under cold water.

Heat 1 tbsp of oil in a wok or heavy skillet over medium-high heat. Add the marinated beef in a single layer and sear for 1-2 minutes until a slight charred crust forms. Flip, cook for 1 more minute, then remove from the pan.

Wipe the wok clean, add the remaining 1 tbsp of oil, and sauté the garlic and ginger for 30 seconds. Pour in the prepared sauce slurry and stir constantly until it bubbles and thickens into a glossy, dark brown glaze.

Return the seared beef and blanched broccoli to the wok. Toss until fully coated in the sauce. Serve immediately over fluffy steamed white rice, garnished with toasted sesame seeds and chopped scallions.