In a small bowl, whisk together the marinade ingredients: soy sauce, rice vinegar, sesame oil and cornstarch.
Pour the marinade over the shrimp in a shallow dish. Let it marinate for at least 15 minutes while you prepare the other ingredients.
In another bowl, whisk together the sauce ingredients: soy sauce, oyster sauce, rice vinegar, brown sugar, and sesame oil to make the sauce. If you prefer a thicker sauce, mix in the cornstarch slurry (cornstarch mixed with equal parts water) and set aside.
Cook the chow mein noodles or egg noodles according to the package instructions. Rinse with cold water and set aside.
Heat half the vegetable oil in a large skillet or wok over high heat. Add the marinated shrimp and cook for 2-3 minutes until pink and cooked through. Remove the shrimp onto a plate and set aside.
Using the same skillet, heat the remaining oil over medium heat. Add the chopped onions and cook for about 2-3 minutes. Add in the garlic and cook for 1 more minute.
Add the sliced mushrooms and shredded cabbage to the skillet and cook until the mushrooms and cabbage are tender and cooked through. The amount will shrink down quite a bit.
Place the cooked noodles into vegetable mixture and evenly combine. Pour the sauce over the noodles and stir-fry everything together until well combined and heated through. Add in the shrimp and sauté for 2-3 minutes. Garnish with sliced green onions and serve.