Remove the ground beef from packaging and sprinkle with salt. Form into 4 equal balls and press each as thinly as possible. Slice onions into thin slivers.
Heat a frypan over a medium-high heat and drizzle with olive oil. Add the thinly sliced onions and a sprinkle of salt. Cook 1-2 minutes to give them a bit of color then turn the heat down to medium-low and add the vinegar and a teaspoon of sugar. Let cook 15-20 minutes until golden and caramelized. If they begin to stick, add a tiny splash of water.
Heat a second frypan over a medium heat and drizzle with olive oil. Add the patties one or two at a time depending on the size of the pan. Cook 3-4 minutes then flip, press down with the spatula and cook a further 1-2 minutes. Top each with a slice of cheese and add a lid to the pan. Once the cheesy is melty, remove to a plate and repeat with the other patties.
Spray each side of the bread with cooking spray and toast in a dry fry pan over a medium heat until browned. Repeat with all sides. NB: if you’re stretched for time, just toast them in the toaster.
To assemble the patty melts: Top one piece of toast with a few spoons of the sauce, top with onion then a patty and press another toast on top. Slice in half and serve.
To make the sauce: Place mayonnaise, dijon, lemon juice, garlic and onion powders, salt and black pepper in a bowl and mix until combined.