Start with some dark bread. I use Oroweat Schwartwalder Dark Rye. (I do not work for Oroweat. I just really love this bread.) Put mayo on one slice, mustard on the other. I would advise against a boring yellow mustard -- I'm currently using a champagne mustard we got in Napa in May, but I've also used a dijon or Gulden's, which are tasty, too.
On one slice of the bread, add the cheese. You can use any type you like, but you're going to want to use at least two kinds. I love a mix of Havarti and Swiss. (The Havarti I'm using right now has dill in it, which is very delicious.) Don't use great horking slabs of cheese -- slice it thinly and kind of alternate it on the bread. Then add some crunch. I like sliced onions and sprouts, but one of those would work alone, too, if you hate onions or hate sprouts. But you have to have something to give a little bite to your tooth. If you enjoy tomatoes (I do), add a few thin slices. Put the other slice of bread on top.
Microwave for a minute or until the cheese is starting to melt. Let cool just enough to pick up and devour.
Results? Joy for your tastebuds.