In a medium bowl, combine the baking soda, light soy sauce, oyster sauce, mirin, white pepper, water, and cornstarch and mix. Add in the thinly sliced sirloin, stir until the beef is evenly coated and glossy. Let it marinate for at least 15 minutes, or refrigerate up to 24 hours if making ahead.
In a separate small bowl, whisk together the sauce ingredients.
Bring a pot of salted water to a boil. Add the chopped broccoli and cook for about 1 minute, just until vibrant green and slightly tender. Drain and set aside.
Now prepare and cook your lo mein noodles—1 minute for fresh, or 1 minute less than package instructions if using dried. Drain and rinse under cold water to stop the cooking. Set aside.
Heat one tablespoon of oil in a large skillet or wok over medium-high heat. Once hot, spread the beef out in a single layer and sear for about 30 seconds per side, or until nicely browned. Remove the beef from the pan and set it aside.
Add another tablespoon of oil to the same pan. Sauté the garlic and ginger for about a minute, until fragrant. Add the broccoli and stir-fry for another minute. Pour in the sauce and let it come to a gentle simmer for a minute or two. Then add the cooked noodles and seared beef. Toss everything together for 2–3 minutes, until the sauce evenly coats the noodles.
Turn off the heat, drizzle in the toasted sesame oil, and give it a final toss. Serve hot and enjoy!