Slice the onion thinly (I prefer french cut, but you can cut however you like).
1/2 medium onion
On medium heat, add oil to a pan. Add in the onion and sauté until translucent.
Add in beef. Continue to sauté until about half the pink is gone.
1 lb shaved beef
Add in mushrooms. Sauté some more.
1 packet shimeji mushrooms
Add simmering sauce. Stir and keep an eye on the pan, moving the ingredients around so it evenly cooks. Keep waiting and stirring occassionally until most of the liquid evaporates.
Remove from heat. Serve over a bowl of freshly cooked rice.
Notes
Use gluten free tamari to make this dish gluten free.