This is a favorite I made up while working in my family's restaurant in New Orleans. I got tired of the same ole' stuff on the menu so had fun with making this. I even served it to my favorite customers if they asked real nice!
First, grill your onions to your liking in a pan with a small amount of oil. Put aside.
Wash lettuce and have your sandwich fillings ready for assembly.
On a hot plate, skillet, etc., place one slice of ham down, followed by swiss cheese, then the other slice of ham (if you use the long slices of ham, they will wrap around the cheese quite nicely). The cheese should be as protected from the pan as much as possible. You want to heat on a medium/high heat and flip this after a minute or two so the top side gets a turn down on the pan. But don't cook this too long as it will make the meat tough and the cheese will melt and leak out all over the place. Just heat until meat is slightly brown on top and bottom and the cheese has mostly melted.
While meat and cheese are cooking, place 2 slices of bread in toaster. When done, add mayo and/or mustard to toast and place ham/cheese on bottom slice of toast. Top with grilled onions, lettuce, tomato, pickle, and sprinkle with salt and pepper. Add top piece of toast, slice sandwich on the diagonal, throw on a plate with some potato chips and get busy (all the while wondering why you didn't go ahead and make that second one while you were at it!) -- back to the kitchen. These things ARE addictive. You've been warned!