¾lbthinly sliced beef (chuck or rib eye)(12 oz, 340 g: Your local Japanese market may sell packages with the name “Komagire”, which includes thinly sliced scraps from different parts and works great for Gyudon. If you have to buy a rib eye or chuck and cut into slices, 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. After you slice, if the piece is too large then cut them in half. I use "Komagire" cut from my local Japanese market and I cut them further into smaller pieces.
Cut the onion into thinly slices and chop the green onion. Set aside.
Heat a large frying pan over medium-high heat and add ½ cup (120 ml) dashi, 1 Tbsp sake, 1 Tbsp sugar, 2 Tbsp mirin, and 2 Tbsp soy sauce.
Cover the lid and bring the sauce to boil. Once the sauce is boiling, add the sliced onions and spread them out. Cover to cook until tender (if you don’t cover, the sauce will evaporate, so make sure you cover the lid).
Top with chopped green onion and pickled red ginger. If you like to add an egg, serve with onsen tamago. 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.