01 - Rinse the rice under cold water until it runs clear. Combine rice and water in a 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 for 2-3 minutes until softened.
03 - In a bowl, combine dashi stock, soy sauce, mirin, sake, and sugar. Pour the mixture into the skillet with onions and bring to a gentle simmer.
04 - Spread sliced beef evenly in the pan. Simmer for 5-7 minutes until just cooked through and tender, skimming foam or fat if needed.
05 - Fluff the cooked rice and divide among serving bowls. Spoon the beef and onion mixture with sauce over the rice.
06 - Top with sliced spring onions and pickled ginger if desired. Serve immediately.