01 - Rinse the rice under cold water until water runs clear. Combine rice and water in a rice cooker or saucepan and cook according to package instructions.
02 - Heat vegetable oil in a large skillet over medium heat. Add sliced onion and sauté until softened, about 2 to 3 minutes.
03 - Whisk together dashi stock, soy sauce, mirin, sake, and sugar in a bowl. Pour into skillet with onions and bring to a gentle simmer.
04 - Add the thinly sliced beef evenly over the sauce and onions. Simmer for 5 to 7 minutes until beef is cooked through and tender, skimming off foam or fat as needed.
05 - Fluff cooked rice and divide into bowls. Spoon beef and onion mixture with sauce over rice. Garnish with sliced spring onions and pickled ginger if desired. Serve immediately.