01 - In a small bowl, whisk together the soy sauce, oyster sauce, hoisin sauce, sesame oil, black pepper, sugar, cornstarch, and cold water until smooth. Set aside.
02 - Bring a pot of water to a boil and cook the chow mein noodles according to package directions until al dente. Drain and rinse under cold running water to halt cooking and prevent sticking. Set aside.
03 - Heat 1 tablespoon of vegetable oil in a large wok or heavy-bottomed skillet over high heat until the oil just begins to smoke. Add the sliced beef in a single layer and stir-fry for about 2 minutes until browned on all sides but still slightly pink inside. Transfer the beef to a plate and set aside.
04 - Add the remaining 1 tablespoon of vegetable oil to the wok. Add the minced garlic, julienned ginger, and sliced onion, and stir-fry vigorously for 1 minute until fragrant and the onion begins to soften.
05 - Add the sliced bell peppers to the wok and continue stir-frying for 2 to 3 minutes, tossing frequently, until the peppers are crisp-tender and have brightened in color.
06 - Return the seared beef and any accumulated juices to the wok. Add the cooked noodles and pour the prepared sauce over everything. Using tongs or a spatula, toss all the ingredients together vigorously for 2 to 3 minutes until the sauce evenly coats the noodles, beef, and vegetables, and everything is heated through.
07 - Transfer to a serving platter or individual plates, scatter the sliced scallions over the top, and serve immediately while hot.