This is my brilliant mother's recipe and it's one I use when I do my shopping. Or when I need something really fast, but totally delicious. It's also one I can't give exact quantities on because it will depend on how many people are in your family. Quantities listed below are a family of 6 - 2 adults and 4 children ages 8 and under.
3/4 to 1 lb. roast beef (from the deli)
3/4 to 1 lb. sliced white cheese from the deli - muenster, provolone, mozzarella, etc.
hoagie or french rolls - one per person
1 lb. sliced mushrooms, optional
1 lg. onion, quartered, optional
If you want a buttery flavor then slice each roll, spread with butter and lay butter side down on a large cookie sheet. Toast at 400 degrees for a few minutes, until butter is melted and bread is barely crisp. You can also just toast the bread without the butter. Flip them over and add a few slices of roast beef and some cheese. Stick back into the oven and bake until the cheese is melted, about 5 minutes.
If you want (and I highly recommend it) you can saute the mushrooms and onions in butter while everything is toasting and then put a large spoonful or two on top of the cheese once it's melted. Or on top of the roast beef, then add the cheese and melt it all. My kids don't like the mushrooms/onions so they eat theirs with meat and cheese only. The two littlest guys (ages 3 and 5) split one.
An alternative to the roast beef is to use some sort of beef sliced super thin, or stir fry meat (make sure you cut it smaller if the strips are very long). Brown the meat in some butter and throw in some garlic powder, onion powder, beef bouillon granules, soy sauce, worchestershire sauce, and Jane's Mixed-up seasoning or salt. This is sometimes cheaper than a pound of roast beef, but takes longer to make.