Season prawns with salt and pepper. Combined all sauce ingredients into a bowl.
Bring a pot of water to boil. Add hot water into udon noodles and stir to loosen it with a chopstick. Once soften, drain noodles into a colander and set aside.
Heat wok with some vegetable oil. Add prawns and cook for 2 minutes or until pink. Remove and set aside.
In the same wok, add in remaining vegetable oil. Add in garlic and onions. Saute for 1-2 minutes till fragrant. Add in the rest of the vegetables and mushroom. Cook for 3-4 minutes or until vegetable has softened.
Add udon noodles, cooked prawns and sauce. Toss together and let it cook for 2-3 minutes until noodles are coated with sauce. Add in green onions and mix to combine.
Portion out on 2 plates. Garnish with remaining green onions and serve hot.