Mix the mentsuyu and water together and set aside.

Peel and butterfly the shrimp along the back, removing the vein. Quickly boil them in salted water with a splash of sake (not included in ingredient list), then drain.

Heat oil and garlic in a frying pan. Add the prepared shrimp, clams, and mushrooms. Sauté briefly, then add the sake. Cover and steam until the clams open.

Add the defrosted udon noodles and stir-fry together. Pour in the mentsuyu mixture and cook until the sauce is mostly absorbed.

Serve on a plate. Generously sprinkle with coarsely ground black pepper and top with chopped green onions.