1Slice the onion into thin half-moons. If your beef isn't already sliced, cut it against the grain as thin as you can, then pull the slices apart so they don't clump.
2Set a large skillet over medium heat. Add the onion, soy sauce and mirin, and stir to coat. Cover and cook for 5 to 6 minutes, stirring once or twice, until the onion is soft and has released its juices into a loose, glossy sauce.
3Uncover and lay the beef over the onions, separating the slices as you go. Simmer for 3 to 4 minutes, nudging the slices around, until the beef is no longer pink and the sauce has reduced slightly.
4While the beef simmers, heat the rice in the microwave according to the packet directions.
5Divide the rice between two bowls. Spoon the beef and onions on top and pour over all the juices from the skillet. Serve hot.