Mix teriyaki sauce with dashi stock in a medium sized saucepan; bring to boil on medium high heat.
Add onions and simmer for 2-3 minutes until soft.
Add thinly sliced beef and cook for 30 seconds or until cooked to your liking.
Serve over rice. I like to add a soft boiled egg and Japanese pickled ginger!
Notes
KEY TIPSBEEF It is key that you get thinly sliced beef! When making this dish, I will purchase the pre-sliced beef from my local Japanese grocery store (Mitsuwa) - it's labeled as 'Kiriotoshi'. If you are buying this from your local grocery store, you can use ribeye or chuck roll - ask your butcher to slice them for you at the thinnest setting (or 1/8").MIRIN You can sub mirin with rice vinegar - but since this recipe has so few ingredients, I highly recommend you use mirin.SAKE You can sub sake with another dry cooking wine (such as white wine or sherry) - but since this recipe has so few ingredients, I highly recommend you use sake.
RECIPE TIPS
TERIYAKI SAUCE Trust me, HOMEMADE is 100% better in this case!
WATCH YOUR SAUCE - You want the sauce to be thick and syrupy, which will happen as it cooks down. HOWEVER, as the water cooks down, there is a higher chance of the sugar burning, so keep this on a simmer and watch it!
You can store this in an airtight container in your refrigerator for up to one week!
OPTIONAL GARNISHES I like to top my Beef Gyudon with a soft boiled egg and Japanese pickled ginger.