1. Cook the rice in water for 10 minutes after it comes to a boil, covered.
2. Slice the beef into thin strips. Blanch in boiling water for a few minutes, then set aside.
3. Heat a skillet over medium heat and mix together sugar, soy sauce, mirin, and sake. When the mixture boils, add the beef and coat it well with the sauce.
4. Before the sauce completely evaporates, set aside the beef and add the 1/4 cube and 100ml of water to the sauce.
5. Add the sliced onion and cover. When it comes to a boil again, add the beef and simmer covered for 5 minutes.
6. Serve the rice in bowls, topped with the beef.