Trim the root of the shimeji mushrooms and separate them into bite-sized pieces. Mince the onion, garlic, and parsley.

Heat olive oil in a frying pan, add garlic, and sauté until fragrant. Then add onion and shimeji mushrooms, cooking over medium heat until softened.

Add clams and water to the pan, mix, cover with a lid, and cook over low heat for 10 minutes until the clam shells open.

Season the mixture with salt and pepper, and simmer over medium heat until the sauce thickens.

Boil water in a pot and cook spaghetti according to the package instructions.

Add the cooked spaghetti to the pan and mix well to coat the pasta evenly with the sauce.