Prepare the broth: In a pot, combine dashi stock, soy sauce, mirin, sake, and sugar. Bring to a gentle simmer over medium heat and keep warm.
Cook the beef topping: Heat oil in a pan over medium heat. Add sliced onions and sauté until softened. Add beef slices and cook until browned. Stir in soy sauce, mirin, sake, and sugar, cooking until the sauce thickens slightly. Set aside.
Cook the udon noodles: If using fresh or frozen udon noodles, cook according to package instructions (usually 1-2 minutes in boiling water). Drain and rinse under warm water.
Assemble the niku udon: Divide the cooked udon noodles into bowls. Ladle the hot broth over the noodles.
Add the toppings: Arrange the cooked beef and onions on top of the noodles. Garnish with sliced green onions and nori strips if using.