Heat oil in a frying pan with medium heat. Add beef and cook until brown. Season with soy sauce and sugar. Turn off the heat. Set aside the beef to a plate and leave the liquid in the frying pan.
Broth
Add dashi to the frying pan and bring to boil. Season with soy sauce, mirin, sugar, and salt. Taste and adjust the seasoning. Set aside.
Put the cooked udon noodles to the serving bowls. Pour the broth over the udon noodles and and serve the stir-fried beef and green onion at the top of the udon noodles. Serve hot.