Meanwhile, clean the mushrooms with a damp cloth and a (soft!) vegetable brush if needed. It is always better to avoid water as the flavor might be altered. Dry them with a kitchen cloth and slice them vertically by a centimeter (scarcely half an inch).
Toss the pasta into the boiling water and cook it according to the package directions for "al dente". A couple of minutes before done, collect some of the starchy cooking water with a cup and set it aside.