Heat the 1 tablespoon of butter or oil in heavy skillet and add your thinly sliced onions and brown very slowly until they are caramelized. Set aside until ready to put burger together.

Season your meat with whatever spices you usually use on your hamburgers and form into oval patties. They are suppose to be thin but I made mine thicker.

Mix the mayonnaise and butter and set aside.  Take one slice of rye and put a slice of Swiss cheese on top then butter the outside with some of the mayo/butter mixture. Do the same with the second piece of rye bread but put the white cheddar on top (you can use all the same kind of cheese if you want) and spread with the mayo/butter mixture.

Heat a flat griddle and add the bread slices, cheese up and start browning VERY slowly. You do not want the bread to get too toasty before the cheese starts to melt. When it looks like you have accomplished this, put the two pieces of cooked bacon on top of one slice of bread/cheese, top with the cooked burger patty then some of the cooked onions and then the other slice of bread/cheese that has been toasting away. (You can add some mustard to one side of the bread if you like.)

Leave this on the griddle for about 5-6 minutes longer, turning every minute or so, just don't get too brown. (You can add some spicy mustard to the burger before putting on the last piece of toasted bread.

Yum, this was delicious -- every bite crunchy and makes my mouth water right now just thinking about the night we had this sandwich ? burger?  You decide.