Slice onion thinly. Cut beef into bite sized pieces. Put dashi, soy sauce, sugar, mirin and sake in a pan. Add onion slices in the pot and simmer for a few minutes. Add beef in the pan and simmer for a few minutes.
Serve hot Japanese rice in a bowl. Put the beef topping on the top of rice.
Top with red pickled ginger (NOT the pink sushi ginger). Some people like red pepper flakes as an added topping.