Turn one section of your griddle at low heat and the other one at hi. If you only have one section, put it at low first.
Melt the butter and cook the onions until they turn brown.
While the onions are cooking, form 2 large balls with the ground beef.
Place the meat balls on the griddle and with a burger press, press the meat down to form a patty.
Season with salt and pepper and cook for about 2-3 minutes or until seared and juice comes out of the surface.
Turn the patty over, season the other side with salt and pepper.
Add 2 swiss cheese slices on top.
Put the griddle lid and cook for about 3 minutes.
Lower your griddle at medium.
Spread mayonnaise on one surface on each bread slices.
Put 2 bread slices on the griddle, mayonnaise faced down, and put a meat patty on each bread slices.
Put some caramelized onions and put the other bread slice on top, mayonnaise facing up.
Flip over each melt.
While the other side of the bread is being cooked, Put about 1 cup of cheddar cheese on the griddle. Make 2 portion, larger than the bread.
While it's starting to melt, Put the bread and patty on top of the cheese.
Make two similar cheese crust with the other cup of cheese, and flip the melt on the other cheese crust.
Lift the melt, so the cheese drop on each side with gravity. Hold for 30 seconds to let the crust cool down and stay in that position. Flip over on a plate.
Cut and serve