Sprinkle the meat all over with salt and pepper. Set aside.
In a small bowl, mix together the cornstarch, baking soda. In another small bowl, add the vegetable oil and water.
Place the cut beef into a large ziplock baggie, or baking dish. Pour the cornstarch mixture and water/oil over the beef. Seal the baggie and use your hands to massage the beef into the cornstarch and water/oil. Place in the fridge for 1 hour, or up to 24 hours.
Combine all of the sauce ingredients into a medium-sized bowl and whisk together until blended.
Heat the oil (¼ cup) in your wok (or skillet) over high heat. Add the garlic and cook for about 30 seconds, until just starting to turn golden.
Carefully add the beef and stir with a wooden spoon until the pinkness is almost gone, about 4 minutes.
Add the onion, red and green peppers, and cook, stirring frequently, for another 2 minutes.
Transfer the sauce to the wok, and stir continuously for 2 minutes, or until the sauce has thickened slightly. Turn off the heat and stir in half of the sliced green onions.
Serve at once with steamed rice or noodles on the side and extra sliced scallions and sesame seeds for garnish.
