Divide the ground beef into four equal portions and gently shape each into a small, round patty about 3 inches in diameter, handling the meat as little as possible for tender results.
Season each patty generously with salt and pepper on both sides, then set aside while you prepare to toast the buns.
Lightly butter the cut sides of the mini buns and toast them in a hot skillet or toaster oven until golden brown and slightly crispy, about 2-3 minutes. Remove and set aside.
Heat a teaspoon of oil or butter in the skillet over medium-high heat until shimmering, then carefully place the patties in the pan. Sear for about 2-3 minutes per side until the edges are crispy and the juices bubble out.
During the last 30 seconds of cooking, place a slice of cheese on each patty and cover the pan briefly to melt the cheese into a gooey layer.
While the patties finish cooking, prepare the sauce by mixing ketchup, mayonnaise, mustard, and pickle juice in a small bowl until smooth.
Assemble each slider by placing a cheesy beef patty on the bottom half of a toasted bun, then topping with pickles, sliced onions, and a dollop of the sauce. Cover with the top bun.
Repeat with remaining patties and buns, then serve immediately to enjoy the crispy edges and juicy centers.
