Preparation

FORM THE PATTIES

1. Mix together beef, 2 tablespoons of the mustard, garlic powder and 1 teaspoon of the worcestershire. Season with salt and pepper. 
2. Divide mixture into 4 balls and press each ball onto a piece of parchment or wax paper. Press meat into a shape similar to the bread but slightly larger (since meat will shrink slightly during cooking).

MAKE THE SAUCE

1. In a separate bowl, whisk together mayonnaise, horseradish and 1 teaspoon of the mustard. Season with salt and pepper. Refrigerate until ready to use. 

MAKE THE ONIONS

1. Heat HexClad Double Burner Griddle over medium. 
2. Add the oil and butter and allow the butter to melt. 
3. Add the onions and a large pinch of salt. Cook, stirring often, until beginning to soften and brown, about 15 minutes (adjust heat if browning too quickly). 
4. Push onions to one side of the griddle and sprinkle with sugar and remaining teaspoon of worcestershire. Continue to cook over low heat until soft and evenly caramelized, 10 to 15 minutes more. Transfer to a bowl. 

MAKE THE MELTS

1. Raise heat to medium-high. Working two at a time, add patties to one half of the griddle. Cook until crisp around the edges and underside is browned, 2 to 3 minutes. 
2. Flip and sprinkle each with a quarter of the grated white cheddar cheese, about 2 tablespoons of the caramelized onions, and 2 slices of swiss cheese. Cook 1 to 2 minutes more.  
3. Butter the other half of the griddle and add 4 slices of bread. Cook 1 minute until underside begins to toast, then spread all slices of bread with the mayonnaise mixture.
4. Top 2 slices of bread with cooked patties, more onions if desired and arugula. Close melts and continue to cook, flipping occasionally, until crisp on both sides and cheese has melted. Transfer to a cutting board. Repeat with remaining patties and bread. 
5. Slice each melt in half and serve with cornichon.