Step 1

First, make the burger sauce. In a small bowl, mix mayonnaise, mustard, relish and ketchup. Cover and place in the fridge for later.

Step 2

Add sliced onion, 2 tablespoons butter, red wine vinegar and worcestershire sauce to a skillet over medium-low heat. Let cook for about 20 minutes with lid on stirring occasionally. When, done transfer onions to a plate.

Step 3

In the meantime, form two hamburger patties and season them with salt and pepper on both sides. Drizzle a little worcestershire sauce on the patties and rub mustard on both sides of the patties.

Step 4

Heat your cast iron skillet or skillet to medium-high heat. Place burger patties on the skillet and cook for about 4 minutes. Flip burgers over and cook for another 3-4 minutes for a medium-well done burger.

Step 5

Butter four pieces of rye bread and place two of the slices butter side down in the same skillet you cooked the onions. Spread burger sauce on the top of all four slices of bread(opposite side of the butter). Then, add a slice of cheddar cheese to the two slices in the skillet. Top cheddar with grilled onions then top onions with a burger patty. Place a slice of muenster cheese on top of the burger patty and then top with the slice of rye bread burger sauce down.

Step 6

Turn skillet up to medium-high heat and cook until bottom rye is nice and golden. Then, carefully flip sandwich over and grill the other side until golden and cheeses are nice and melted.