Add water, sake, soy sauce, mirin, and sugar in a pot over medium heat. Once it starts to simmer, add beef and onion. Turn down to low heat and let it simmer for 5 minutes.
Follow package instructions for Beef Flavor Udon to prepare the udon.
Add udon and soup in a bowl and top it with simmered beef.
Garnish with scallions, add ichimi togarashi, and red ginger to your liking. Enjoy!