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