01 -
Whisk mayonnaise, mustard, ketchup, and garlic powder in a small bowl. Set it in the fridge so the flavors settle while you cook.
02 -
Place a cast-iron skillet or griddle over high heat until it is smoking hot. Add a thin layer of oil and let it shimmer.
03 -
Divide the ground beef into four equal portions and shape them into loose meatballs. Do not pack them tight.
04 -
Lay a small handful of thin onion slices in the pan. Place a meatball on top, cover with a square of parchment paper, and press it flat with a sturdy metal spatula. Get it thin, about 1/4 inch, so the edges crisp. Peel off the parchment and repeat.
05 -
Let the patties cook for two to three minutes undisturbed. The onions underneath will brown and fuse into the beef.
06 -
Flip each patty, season the cooked side, and lay a slice of American cheese on top. Cover the pan briefly to melt the cheese.
07 -
While the cheese melts, press the buns cut-side down into the hot skillet until they turn golden and crisp.
08 -
Spread sauce on the toasted buns, add the patties, and top with pickles, lettuce, or tomato if you like. Serve right away while the edges are still crisp.