Step 1

Place a medium skillet over medium heat. Halve and thinly slice the onions and add to the pan along with 2 tablespoons of the butter. Season with salt and pepper and cook until the onions soften slightly and pick up a little color, 5 minutes. Turn the heat down to medium-low and continue to cook until the onions are well-caramelized, 15 to 20 minutes. If the onions start to catch at any point, just add a few tablespoons of water to deglaze the pan. When the onions are caramelized, stir in the vinegar or lemon juice and transfer the onions to a dish. Wipe out the pan and set aside.

Step 2

Meanwhile, combine the turkey, mayonnaise, garlic, Worcestershire, breadcrumbs and 1/4 teaspoon of pepper in a large bowl. Divide the mixture into 4 equal portions and form into patties roughly the same shape as the bread. Season the patties well with salt.

Step 3

Place the skillet back over medium heat and melt in 1 tablespoon of butter. Add the patties into the pan and, using a spatula, squash them down slightly. This will ensure a good golden-brown crust on the patties and allow for a bit of shrinkage that, like with any ground meat patties, is sure to happen. Cook until golden and cooked through to 165 degrees F, 3 to 4 minutes per side. Set the patties aside and wipe out the pan.

Step 4

To assemble the sandwiches, use the remaining 3 tablespoons butter to butter one side of the bread slices. Add half of the bread to the pan, butter-side down, and turn the heat down to medium-low. Divide over the cheese and onions and top each with a patty. Place a piece of bread butter-side up on top of each and cover the pan with a lid. Cook until golden on the first side and the cheese has melted then remove the lid, flip and continue to cook until golden brown.