Smash Burger

Make the Sauce - In a small bowl, combine the mayonnaise, mustard, and relish. Cover and refrigerate until ready to serve.
Form the Patties - Divide the ground beef into four equal portions, 4 ounces each. Loosely form the beef into rounds. Generously season both sides with salt and pepper.
Heat the Pan - Add the vegetable oil to a large 12-inch cast iron skillet or stainless steel pan. Use a paper towel to spread it into an even layer. Heat the pan over medium heat for 2 minutes. Increase the heat to medium-high right before adding the patties.
Cook the Burgers - Carefully add 1 patty at a time to the pan. Immediately use a metal spatula or burger press to flatten the patty down to ½-inch thick and 4-inch wide. Continue adding and flattening the remaining patties. If needed, cook in two batches, depending on pan size.
Cook without moving until a golden brown crust develops, about 1 1/2 to 2 minutes. Use the spatula to flip and cook for about 1 minute, then add the cheese until melted, about 1 minute.
To Serve - Toast the burger buns and spread on the sauce. Add the pickles, lettuce, tomato, and onion, then the patty.