Cut beef, green onion, onion, shiitake mushrooms, udon noodles, and tofu into bite-sized pieces.
Add cooking oil to a pan and stir-fry the beef.
Add the green onion, onion, and shiitake mushrooms and continue stir-frying.
Season with soy sauce, sugar, mirin, and sake, then add tofu and simmer.
Add udon noodles and boil together with the other ingredients.
Serve the finished sukiyaki udon in a bowl.