Thaw frozen udon noodles by soaking in hot water for a few minutes. Loosen the noodles with chopsticks or tongs, then drain the water completely and set the noodles aside.
Lightly season the shrimp with salt and pepper, then set aside.
In a bowl, combine all the sauce ingredients. Mix until well blended, then set aside.
Heat 1 tbsp of oil on a nonstick pan. Add the shrimp and cook until both sides are no longer pink. Transfer to a separate plate immediately.
In a large pan or wok, heat up 2 tbsp of neutral oil, then add the onions and garlic and stir fry for about a minute.
Add the cabbage and mushrooms and stir fry for another 3 minutes, or until the vegetables are cooked to your liking.
Add the thawed udon noodles, quickly followed by the sauce. Mix until ingredients are coated with the sauce, then top with cooked shrimp. Briefly mix until all ingredients are well incorporated, then top with optional scallions.
Enjoy!