This recipe upgrades the traditional patty melt by adding tangy caramelized onions and crispy prosciutto to the equation.
Cook the onions in a medium pan over medium-low heat with a little olive oil and red wine vinegar. Season with salt and pepper. Let the onions cook for about 15-20 minutes, stirring occasionally, until soft and slightly caramelized. Once ready, set the onions aside on a dish, and in the same pan cook the prosciutto. Because the slices are thin, it should only take a few minutes on each side. Once the prosciutto is crispy, set aside on a paper towel-lined plate.
Meanwhile, in a medium-sized mixing bowl, add the ground beef, mustard, and Worcestershire sauce. Mix together with hands until the ingredients are integrated,but don't over work the meat. Season generously with salt and pepper and mix again. Form the meat into four 4-6 ounce patties (for this type of burger I prefer the patties to be flatter rather than fatter, it fits the bread better).
Cook the burgers in a grill pan over high heat about 2-3 minutes a side, depending on desired doneness. While the burgers are cooking, toast the bread. About a minute or so before the burgers done add a slice of cheese on top of each patty. (To help speed along the process, I like to hold a large pot cover over the pan so that the steam melts the cheese.)
To assemble, place a patty on a slice of the bread, add some onions and a piece of the crispy prosciutto and top with second piece of bread.