I like to cook my onions first to get them soft. Heat a pan with some sunflower oil and sautee the onions for 2 mins.
Add the beef and sautee till it's semi-cooked.
Add the sake, mirin and soy sauce and some water (depending on how dry you'd like it to be)
Let it simmer in low heat for few seconds.
Add the brown sugar and stir.
Pour the beaten egg all over the beef and cover for 2 seconds just to let the egg cook but not all the way. Turn off the heat.
Ladle some rice into a bowl. Pour the contents of the beef and egg on top.
Garnish with sprinkling of shichimi pepper and shredded seaweed.