In a small bowl, mix together the mayonnaise, Dijon mustard, ketchup and sweet relish. Season with salt and pepper to taste.
Sauce can be prepared a day in advance and refrigerated.
Make Burgers:
Divide meat into quarters. Shape each quarter into a ball. Do so as efficiently as possible taking care not to overwork the meat. Return to the refrigerator until you’re ready to cook the burgers.
Preheat a cast iron frying pan or griddle to medium.
Depending on the size of your pan or griddle, you may need to prepare burgers in batches. (We do two at a time and transfer the cooked burgers to a plate and cover with foil.)
Add burgers to pan or griddle, put a sheet of parchment paper or wax paper on top of the burger, and immediately flatten using a hamburger or panni press, stiff spatula, bottom of a heavy saucepan or anything that will flatten them into a flat, thin patty.
Remove parchment paper or wax paper and salt and pepper each burger.
Cook for 2 minutes on the first side or until seared.
Using a flexible spatula (to ensure you retain any caramelized parts), flip burgers, salt and pepper, and top with a cheese slice.
Cook for 1 to 2 minutes on the second side.
If using a bun, spread some sauce on each side of the bun. Add the first burger with cheese to the bottom of the bun, top with some more sauce, add a tomato slice, dill pickle chip, red onion slice and/or lettuce, top with the second burger, repeat, and top with bun.
If you’re not using a bun, top the first burger with sauce, add a tomato slice, dill pickle chip, red onion slice and/or lettuce, top with the second burger and repeat.
Serve immediately.
Notes
* The sauce is great for dipping French fries (regular or sweet potato) or tater tots, so you may want to prepare extra. * I like a piece of cheese on each burger, but you can just put a slice on the first burger and it will still taste great.
Recipe by Riegl Palate at https://www.rieglpalate.com/smash-burger/