Boil the buckwheat noodles and rinse them in cold water.
Cut the beef into bite-sized pieces and prepare the mushrooms.
Stir-fry the beef with sesame oil in a pot.
Add the mushrooms and stir-fry together.
Pour in water and bring to a boil.
Season with soy sauce for soup and salt.
Pour the broth over the prepared buckwheat noodles and top with mushrooms and beef.