To season the shrimp: In a medium bowl, season the shrimp with the salt and white pepper. Mix until evenly coated.
To make the sauce: In a small bowl, whisk together the soy sauce, dark soy sauce, oyster sauce, and water until well combined.
To cook the noodles: Bring a large pot of water to a boil. Add the lo mein noodles and cook for about 2 minutes, or until just al dente. Drain and rinse under cold water to stop the cooking process. Drain well.
To cook the shrimp: Heat 1 tablespoon of cooking oil in a wok or large skillet over high heat. Add the shrimp and garlic and stir-fry for about 1 minute, or until the shrimp turn pink. Remove and set aside.
To cook the vegetables: Add 1 tablespoon of cooking oil to the same wok. Stir-fry the onion, shiitake mushrooms, and cabbage for about 30 seconds, or until just softened. Remove and set aside.
To finish the lo mein: Add the remaining 1 tablespoon of cooking oil to the wok. Add the cooked noodles and stir-fry for about 1 minute. Return the shrimp and vegetables to the wok, then pour in the sauce. Toss until the noodles are evenly coated. Stir in the bean sprouts and scallions and cook for about 30 seconds. Serve immediately.