In a large sauce pan add your cooking oil, once it's heated, add your onions. Cook for approximately 10 minutes till they soften and become translucent.
Once the onions have cooked in the same pan, increase your heat and add in the beef. Brown until all of the meat is no longer red, season with salt and pepper and then let it stay warm on low heat.
If you're making a cheese sauce, in medium sauce pan add your pub cheese and condensed milk. Stir for about 5 minutes at medium heat so you have a perfectly smooth gooey cheesy mixture. It should look like cheese whiz. If you want to just use cheese whiz instead, just simmer however much you want on the sandwich in a pot or nuke it in the microwave for 45 seconds until it's nice and melty.
Assemble your sandwich on the roll, meat first then the cheese if you're going whiz. If you're going provolone or american, melt the cheese on the steak prior to putting on the bread. Enjoy!