In a pot, combine the broth, soy sauce, mirin, and sugar. Bring to a boil, then turn off the heat and set aside.
In a pan, drizzle some oil and cook the onions until translucent. Add the beef and sauté until browned. Stir in soy sauce, mirin, sake, sugar, and salt to taste.
Boil the frozen udon noodles according to the package instructions.
To serve, place the cooked udon noodles in a bowl. Top with the beef mixture, pour over the broth, and garnish with green onions, a poached egg, or nori.