1 thumb size ginger, peeled and sliced into thin strips
8–10 mushrooms, sliced into strips
2 tablespoons vegetable oil
2 1/2 tablespoons granulated sugar
2cups dashi broth
1/2 cup soy sauce, plus an additional 2 tablespoons
1 teaspoon sake (optional)
Put beef in the freezer for about 15-20 minutes (this makes it easier to slice).
In a large deep pan, turn the heat to medium high and add oil and ginger strips. After one minute, add onions and cook for 3-4 minutes or until they are translucent. Add mushrooms and cook for 3 minutes. Add the dashi broth, soy sauce, sugar, mirin and sake, and bring to a boil.
Take beef out and with a knife, slice the beef as thinly as you can against the grain. Add to the pan, lower the heat a little and cook for about 10 minutes.
You will be left with quite a bit of sauce/soup. This is going to be very strong in flavor so only use a teeny bit to drizzle on top.
This is best served over a bowl of Japanese rice and seasoned with Ichimi or Shichimi (both are red pepper flakes).