Patty Melt

Peel and slice onions in half, then from pole to pole, in 1/4" slices. Over medium heat, melt the butter in a large skillet and add the onions, stirring frequently, adding more butter if needed, reduce heat a bit off they start to burn. 

While the onions and frying divide the beef into 3-or 4 equal portions and roll each into a ball. Slice dill pickles lengthwise, and grate enough cheese to add to each side of the sandwich.

After the onions are crispy and browned transfer to a bowl or plate. Scrape the pan out and increase heat to medium high. Add the beef, one ball at a time and flatted each one with a press or spatula until very thin, just over a 1/4" or as close as you can. cooking two or three at a time if there's room in the pan. Press down as the cook, flip and press after 4-5 minutes, and repeat on the other side. Set aside on another plate.

Spread mayonnaise on one side of each piece of bread. Reduce heat to medium. Add slice, mayo side down in the pan, add a later of cheese, then a layer of onions, top with a patty, 2-3 pickle slices, more cheese, and another slice of bread, mayo side up. Add another sandwich if there's room. Fry for 2-3 minutes, after the bottom is crispy and brown. CAREFULLY flip, press down with a spatula to flatten a bit and cook for 2-3 minutes more, or longer if needed. 

Side aside and let sit for a few minutes to cool down and let the cheese continue to melt. Serve with ketchup or some more mayonnaise.