To season the shrimp, in a bowl, mix the shrimp with the salt, white pepper, and cornstarch until evenly coated.
To make the sauce, in a bowl, mix the soy sauce, dark soy sauce, oyster sauce, and chicken stock until combined.
To cook the udon noodles, in a pot over high heat, bring water to a boil and cook the udon just until the noodles separate, then rinse briefly in cold water and drain well.
To stir-fry the shrimp, in a wok over high heat, add 1 tablespoon of cooking oil, then add the shrimp and garlic and cook for about 1 minute until the shrimp just turns pink, then remove.
To stir-fry the vegetables, in the same wok over high heat, add the remaining cooking oil and cook the onion and cabbage for about 1 minute until slightly softened, then remove.
To finish, in the wok over high heat, add the udon noodles and stir-fry until heated through, then return the shrimp and vegetables, add the sauce, and toss everything together until evenly coated, then add the scallions and bean sprouts and toss briefly before serving.