¾lbthinly sliced beef (chuck or rib eye)(12 oz, 340 g; your local Japanese market may sell packages labeled “komagire,” which includes thinly sliced scraps from different parts and works great for Gyudon; for better quality meat, use "shabu shabu" beef; if you have to slice the meat yourself, follow my tutorial)
If you cannot find sliced beef in your local grocery store, freeze a block of fresh chuck or rib eye for 1-2 hours and slice. See this post for detailed instructions. If the pieces are too large after slicing, then cut them in half. I use sliced beef labeled "komagire" from my local Japanese market and cut the slices further into smaller pieces.
Cut the onion into thin slices and slice the green onion into thin rounds. Set aside.
Heat a large frying pan over medium-high heat and add the dashi, sake, sugar, mirin, and soy sauce.
Cover the pan with a lid and bring the sauce to a boil. Once the sauce is boiling, add the sliced onions and spread them out in a single layer. Cover to cook until tender (make sure you cover the pan, otherwise the sauce will evaporate).
Divide the steamed rice into individual serving bowls. Serve the simmered meat and sauce over the steamed rice.
Top with the sliced green onions and pickled red ginger. If you'd like to add an egg, serve with an onsen tamago on top. Alternatively, you can pour beaten egg over the meat when it’s almost finished cooking in the pan (see how I do it in my other Gyudon recipe).
You can keep the leftovers in an airtight container and store in the refrigerator for up to 3-4 days or in the freezer for a month.