I love steak ums, and last night I needed a quick dinner for Cor, Jeff, and I to eat between packing up Cor’s life. I don’t buy the packaged ones, I just make them as follows.
You need:
1 package thin steak (usually like 1.5 lbs. This is enough for 3-4 people, or roughly 2 women and 2 large hungry men)
1 package mushrooms (white are good)
1 onion (yellow or red or white)
some cheese (any kind)
1 french baguette
Saute the onion for about 2 minutes, then add mushrooms, continue to saute.
Cook the steak in a pan with some oil or Pam, add the cheese.
Cut the bread. Stick a piece of steak (or two!) on a section of bread, add sauteed mushrooms and onions. Maybe add some ketchup, and devour.
(We made smashed potatoes and green beans to go with it. Yum!)
Providential Life 
I would disagree that “any” cheese could be used and would instead encourage you to use only the most processed cheese on earth” Land O’ Lakes white American. Let me know if you need some, it was on sale and Jon let me get a whole pound (you better hurry up, it will be gone within the week). Also–what kind of steak do you use??
I used Mexican cheese, and it was wonderful. You can also use cheddar, and Swiss is good, too! I might snag some of your cheese, though, white American is delicious. Meanwhile, I use thin-sliced steak from the regular grocery store.