For the Marinade/Sauce:
For the Stir-Fry
Marinate beef, prepare sauce, prep ingredients:
Combine beef, 1 tablespoon soy sauce, and 1 tablespoon Shaoxing wine or dry sherry, and 1 tablespoon of sugar in a bowl and toss to coat. Let marinate for at least 20 minutes at room temperature and up to 3 hours in the fridge.
Meanwhile, add corn starch to a cup and combine with remaining soy sauce stirring with a fork to form a slurry. Add remaining Shaoxing wine or dry sherry, chicken stock, oyster sauce, sesame oil, remaining sugar, and pepper to taste. Set aside.
Combine bell peppers and onions in a bowl and set aside.
Combine garlic, ginger, and green onions in a bowl and set aside.
Cooking:
Work with 1/2 of the ingredients at a time to maintain heat in skillet or wok.
Heat 1 tablespoon oil in a wok or skillet over high heat until smoking. Add half of beef and cook while stirring and tossing until lightly seared but still pink in spots, about 1 minute. Transfer to a large bowl. Repeat with 1 more tablespoon of oil and remaining beef, adding beef to same bowl.
Wipe out wok or skillet if needed. Repeat with 1 more tablespoon oil and half of peppers and onions, cooking until the onion is softened and vegetables are crisp-tender. Transfer to bowl with beef. Repeat with remaining oil and peppers and onions.
Return to high heat and add the contents of bowl back to the wok or skillet, along with the garlic/ginger/scallion mixture. Cook about 30 seconds, tossing, until fragrant. Stir the sauce and add, stirring until just thickened, about a minute or so.
Transfer to a platter to serve.