Slice the flank steak against the grain into thin strips about 1/4-inch thick. In a medium bowl, combine the beef with 2 tablespoons soy sauce, 1 tablespoon cornstarch, and 1 teaspoon sesame oil. Mix thoroughly and let it sit for at least 10 minutes while you prep the other ingredients.

In a small bowl, whisk together 3 tablespoons soy sauce, 2 tablespoons oyster sauce, 1 tablespoon hoisin sauce, 1 teaspoon sugar, 1 teaspoon sesame oil, and 2 tablespoons water. Set this aside within easy reach of your stove.

Bring a large pot of water to a rolling boil and cook the lo mein noodles according to package directions, usually 3 to 4 minutes for fresh noodles. Drain them in a colander and immediately toss with 1 tablespoon vegetable oil to prevent sticking.

Heat your wok or large skillet over high heat until it just begins to smoke. Add 1 tablespoon of vegetable oil and swirl to coat the surface.

Add the marinated beef in a single layer and let it sear without stirring for 1 to 2 minutes until browned on one side. Flip the pieces and cook for another minute until just cooked through, then transfer to a clean plate.

Add another tablespoon of oil to the hot wok. Toss in the minced garlic and ginger, stirring constantly for about 15 seconds until fragrant but not burned.

Add the sliced onion to the wok and stir-fry for 1 minute until it starts to soften. Add the cabbage, carrot, and bell pepper, and continue stir-frying for 2 to 3 minutes until the vegetables are crisp-tender.

Return the cooked beef to the wok along with any accumulated juices. Add the cooked noodles and pour the prepared sauce over everything. Use tongs to toss everything together vigorously for 1 to 2 minutes until the noodles are evenly coated and heated through.

Add the green onion pieces and toss for a final 30 seconds. Taste and adjust seasoning if needed, adding more soy sauce for saltiness or a splash of sesame oil for aroma. Transfer to serving plates immediately while everything is hot and glossy.