My own invention. Hanger steak, or bistro steak, is a very tender cut, of which each cow has only one. If you don't have demi-glace, you can use 1 cup of beef broth plus 1 tbsp of a beef base (a bouillon-like paste).
Soak porcini mushrooms in enough very hot water to just cover. Soak at least 20 minutes.
Melt butter in skillet. Add onion and garlic and saute until tender. Remove mushrooms from liquid (reserve liquid) and dice. Add to skillet with red wine and port wine; bring to a boil and reduce volume by half.
Strain mushroom liquid through a coffee filter to remove grit. Pour sauce through a sieve; discard solids and return sauce to pan. Add mushroom liquid. Whisk in demi-glace. Adjust seasoning with salt and pepper, if needed. Keep warm over low heat.
Sprinkle steak with salt and pepper on both sides. Broil 4 minutes on each side for medium-rare (128F internal temperature). Let stand 5 minutes. Slice thinly across the grain and serve with sauce.