Roast beef, sliced as you like
Hard rolls (I like a whole wheat bolilio)
Onion, halved then sliced into thin slices (I prefer sweet onions)
Portobello mushrooms, if desired, (Sliced)
(I usually use the Crock Pot for the meat/ broth, but it is easily done in any pot.)
Sprinkle with one envelope of dry onion soup mix.
Simmer over low heat.
In a heavy skillet, cook the onion slices in a small amount of olive oil, separating them as you go. I like to cook them until they are brown and caramelized.
Slide the onions (and mushrooms!) to one side of the skillet, and grill the bread halves in the olive oil.