Boil about 12 cups or 3 littler of water in a big pot and add about 3 Tbsp salt (not in the ingredients). Cook spaghetti according to the instructions on the package.

Heat a frying pan at medium heat. Add bacon pieces and cook until browned and somewhat crispy. Add minced garlic and stir. Add Shimeji mushrooms and sliced onion and cook until tender. Add into the pan a quarter cup of boiling water from the pasta, the soy sauce, Mirin, and Sake, and stir well. Add butter in the sauce. Put the cooked spaghetti into the sauce, then add green onions. Stir for a couple of minutes to coat the spaghetti with the sauce.

Serve on a plate, and top with thinly cut Oba leaves and seaweed.