Mix the soy sauce, mirin, sake, brown sugar, crushed garlic and black pepper in a ziplock bag.
Cut the beef steak into cubes and put them in the ziplock bag with the seasonings and marinate them for at least 30 minutes in the fridge.
In a heated cast iron pan, add 1 tbsp of oil and sear the beef cubes till it has a nice golden brown crust.
Add the butter in and flip the beef cubes over.
Continue searing the other side till it turns golden brown.
Remove from pan and arrange the beef cubes on a bowl of steamed Japanese rice.
Add the onsen egg on top of the beef cubes.
Garnish with spring onions if desired.