Boil mirin and sake on high heat for 2 minutes, then mix them in a bowl with soy sauce and sugar.
Stir-fry the sliced onions and garlic over medium-high heat until fragrant.
Once the onions are soft, add butter and remove the garlic. Add the sauce from step 1 and water. Bring to a boil, then taste and adjust seasoning
Add the thinly sliced beef and cook over medium heat for 2-4 minutes, stirring well. Evenly distribute the beef and buttered onions over 2 bowls of rice
You can top the beef with a raw egg or onsen egg, then drizzle with an appropriate amount of sauce to serve
Video
Notes
Ingredients - Click to enlarge the imageTips and notes1- Butter and garlic are my personal additions, which enhance the flavor. However, in traditional preparation, these two ingredients are not used. For a traditional taste, you can omit them2- In this traditional sauce recipe, the level of saltiness may vary depending on personal taste. After adding the water into the sauce mixture, you can taste and adjust the seasoning. If it's too salty, you can add more water and sugar.3- How to make the onsen eggs is in the video for reference4- You can use higher quality thinly sliced beef instead of fatty beef slices