Firm the beef in the freezer for 15 to 20 minutes, then slice it as thin as you can against the grain. Thin slices cook in seconds and stay tender, which matters a lot with lean sirloin.
In a wide skillet or shallow pan, combine the dashi, soy sauce, mirin, sake, sugar, and grated ginger. Add the sliced onion and bring to a simmer over medium heat. Cook for 5 to 6 minutes until the onion turns soft and translucent.
Add the beef, spreading it out so the slices separate. Simmer gently for 4 to 5 minutes, turning once. Skim off any gray foam that rises. Do not let it boil hard or the lean beef will tighten up.
Taste the broth and adjust with a splash more soy or a pinch of sugar. Divide the hot rice between two deep bowls and spoon the beef, onion, and a little of the broth over the top.
Make a small well in the center and slide in a raw egg yolk, or top with a soft-poached egg. Scatter with scallions and pickled red ginger and serve straight away.