Slice 1 pound flank steak against the grain into thin strips. If needed, freeze for 30 minutes first for easier slicing.
In a bowl, combine beef with 1/4 teaspoon baking soda, 1 tablespoon soy sauce, 1 tablespoon sesame oil, 2 tablespoons Shaoxing wine, and 1 teaspoon cornstarch. Mix well and marinate for 15 minutes.
Whisk together 2 tablespoons soy sauce, 2 tablespoons oyster sauce, 1/2 cup chicken broth, 1 teaspoon sugar, and remaining cornstarch in a small bowl. Set aside.
Heat 1 tablespoon vegetable oil in a large skillet or wok over high heat. Add beef in a single layer and sear for 2 minutes until browned. Flip and cook 1 minute more. Remove to a plate.
Add remaining 1 tablespoon oil to pan. Add 3 cloves minced garlic and 1 tablespoon ginger. Stir for 30 seconds until fragrant.
Add broccoli florets and a splash of water. Cover and cook 2 minutes until bright green and crisp tender.
Return beef to pan. Stir sauce and pour over everything. Cook 2 to 3 minutes, stirring constantly, until sauce thickens and coats beef and broccoli.
Serve immediately over cooked white rice. Garnish with sesame seeds and sliced scallions.