Toss the sliced beef with 1 tbsp cornstarch, 1 tbsp soy sauce and the baking soda. Let it sit for 15 minutes at room temperature. Rinse nothing, just move on to the sauce.
Whisk the oyster sauce, 2 tbsp soy sauce, Shaoxing wine, sugar, 1 tbsp cornstarch, 1/2 cup water and sesame oil in a bowl until no cornstarch lumps remain.
Heat a large wok or skillet over high heat until it just starts to smoke. Add 1 tbsp oil, then the broccoli. Stir-fry 1 minute, add 2 tbsp water, cover and steam 2 minutes until bright green and barely tender. Tip it onto a plate.
Wipe out the pan, return it to high heat and add 1 tbsp oil. Cook the beef in two batches in a single layer, 60 to 90 seconds per side, until browned but still slightly pink inside. Move each batch to the plate with the broccoli.
Add the last 1 tbsp oil, garlic and ginger. Stir for 20 seconds, then pour in the sauce. Simmer 1 minute, stirring, until glossy and thick enough to coat a spoon.
Return the beef and broccoli with any juices. Toss for 30 seconds to coat and finish cooking the beef. Turn off the heat.
Divide the rice among four containers, top with the beef and broccoli, and sprinkle with sesame seeds and scallions. Cool uncovered for 15 minutes before lidding and refrigerating.