Season steak with salt and pepper on each side.
using a mallet, meat tenderizer or rolling pin, pound out the steaks so they are about 1/2 inch thick.
Place steak into a zip lock bag or container and add dark soy sauce and mix steak around so it's nicely coated by the sauce.
Let it marinade 30 minutes on a counter.
In a large cast iron pan, add butter and olive oil and heat on high heat, when the butter is melted and hot.
Add steaks and sear quickly on each side then remove and set aside.
Add garlic and mushrooms to the hot pan, reduce to medium heat and stir until the garlic and mushrooms start to soften, should only take 1-2 minutes.
Add chicken stock, salt and Dijon mustard and any liquid on the steak plate.
Simmer for a minute or so until a thin sauce forms.
Add the steak back to the pan and coat nicely on each side with the sauce.
Remove steak from pan, pour the pan sauce over the top and let it sit for 3-4 minutes to rest, then serve, spooning on more sauce as desired.
Top with French fried onions and bleu cheese if desired.
Enjoy!