This is what a patty melt should be. Griddled patties, sauteed onions, a delicious sauce and of course, cheese. And thick bread. You have to have thick bread or you’ll end up with sandwich sadness.
Fire up a grill or heat a skillet over medium-high heat.
Season the burger with salt, pepper and garlic powder and cook until done.
Meanwhile, melt 1 tablespoon of the butter in a medium skillet over medium-high heat and add the onion. Season with salt, pepper and garlic powder and saute until softened.
Combine the mayo, garlic and mustard and slather on one side of each slice of bread.
Melt remaining 2 tablespoons of butter in a large skillet over medium-high heat.
Add the bread slices, mayonnaise mix side up, and swirl around to coat in the butter.
Add one slice of Swiss to two of the bread slices, and one slice of cheddar to the other.
Assemble sandwich by placing cooked patties on top of the two bread slices with Swiss, add the onions and then the top bread slices.