In a large pot, bring water to a boil and cook the udon noodles according to the package instructions. Drain and set aside.
In another pot, heat the sesame oil and sauté the garlic and onion until fragrant.
Add the seafood and cook until they are slightly firm.
Add the stock, soy sauce, mirin, and dried seaweed. Bring to a boil and let it simmer for about 10 minutes.
Add the cooked udon noodles and let it simmer for another 5 minutes.
Season with salt to taste.
Serve hot, garnished with spring onions.