For easier slicing, consider freezing the meat for 30 minutes before preparation.
Slice the steak against the grain into ¼-inch-thick strips. Place the strips in a bowl, add baking soda, and massage it into the meat for a minute.
Add the rest of the marinade ingredients (soy sauce, sugar, and cornstarch). Toss to coat and set aside. You can cook it immediately or let it marinate for 30 minutes on the countertop.
Core and de-seed the bell peppers, then thinly slice them into ¼-inch pieces, along with chopping the onion (pole to pole).
Peel and mince the garlic and ginger.
Combine all the sauce ingredients (excluding the cornstarch slurry ingredients) in a small bowl, whisk well, and set aside.
In a separate small bowl, mix the cornstarch and 1 tablespoon water until lump-free.
Heat a wok or a large skillet over high heat until lightly smoking. Add 1 tablespoon of oil, swirling to coat.
Add half of the beef to the pan and cook, untouched, until well-seared (about 1 minute). Continue sautéing until lightly cooked with just a few pink spots (about 1 minute). Transfer to a bowl.
Wipe out the wok with paper towels and repeat with another tablespoon of oil and the remaining beef.
Once all the beef is cooked and set aside, add the remaining tablespoon of oil. Stir-fry the peppers and onion until tender-crisp and lightly charred in spots (1-2 minutes).
Add the minced garlic and ginger, stir, and return the beef to the wok, stir-frying for 30 seconds.
Pour in the bowl of sauce ingredients and a splash of the cornstarch slurry. Stir-fry until the sauce thickens and the beef cooks through (about 30 seconds).
Adjust the consistency by adding more cornstarch slurry if needed or water (or beef broth) if the sauce is too thick.
Remove from the heat, garnish with sesame seeds, and enjoy!