Begin by dividing the ground beef into 3 oz (85g) portions and rolling each into a ball. Place the balls between two sheets of parchment paper and gently flatten them with a mallet or your palm until they are about ¼ inch thick.

In a medium bowl, combine mayonnaise, sour cream, ketchup, and mustard, mixing well for a creamy, flavorful sauce. Add salt and pepper to taste.

Heat a cast iron skillet or griddle over medium-high heat. Season one side of the shaped patties with salt and pepper, then place them seasoned-side down onto the sizzling skillet. Cook for 1-2 minutes until a golden-brown crust forms, then flip and cook for an additional 30 seconds to a minute.

As soon as you flip the smash burgers, place a slice of cheddar cheese on top, allowing it to melt. Transfer the patties to a plate once melted.

Toast the brioche buns until golden. On the bottom half of each bun, spread a generous spoonful of sauce. Layer fresh lettuce, tomato slices, and onion rings before stacking the patties on top.

Finish the burgers by adding crispy pickles on top and another drizzle of sauce. Place the top bun gently onto each burger and serve immediately.