I first became acquainted with the Monte Cristo at a local eatery, Early Edition, in Manhattan, Kansas. You should check it out if ever in the area, very yummy. Though I will admit a French Toast sandwich did not appeal to me at the time. However, over Thanksgiving weekend I was staying at a friend's place and he made one with some leftover French Toast, it was all I could do not to eat his entire sandwich!
So when I had some French Toast leftover from brunch, I had to try it! Verdict? Best use of leftover French Toast ever! Though, probably not worth making French Toast just to make a Monte Cristo.
Cheese (swiss is traditional but mozzarella or provolone likely as good, I would stay away from anything yellow)
2 slices prepared French Toast
Jam (I believe blueberry is the traditional)
Make your sandwich and grill on low heat, until warm throughout and cheese is melted.