Prepare the clams by soaking them to remove sand and scrubbing the shells.

Trim the root of the shimeji mushrooms and slice the onion thinly.

Boil water in a pot, cook the spaghetti according to package instructions, then drain.

Heat olive oil in a frying pan, add onion, shimeji mushrooms, and grated garlic. Sauté until the onion softens.

Add clams and sake to the pan, cover with a lid, and cook until the clams open.

Add the cooked spaghetti, soy sauce, butter, and salt and pepper to the pan. Stir-fry to combine and evaporate excess liquid.