This is a quick and simple version so that you can enjoy a French Dip sandwich at home.
I use an Au Jus seasoning packet (in the store with all the gravy, stew, chili seasoning packets). Prepare according to package directions (just mix with water and heat it).
Add thinly sliced deli roast beef to the au jus and simmer just until it's heated.
Meanwhile, slice up an onion and cook in a little olive oil until caramelized.
I like a nice firm submarine roll - toast it under the broiler a minute or two. Top both cut sides with cheese (Provolone this time, Swiss sometimes) and a little Italian seasoning, broil until cheese is melted.
Top one side of cheese coated bread with roast beef (pull out of Au Jus with tongs) and then caramelized onions. Top with other piece of cheese coated bread. Pour extra Au Jus into cups for dipping.
The packet of Au Jus makes enough for about 6 sandwiches. I like about 1/4 pound of meat per sandwich.