An In-N-Out style burger with a juicy patty, fresh lettuce, tomato, and special sauce on a toasted bun is a West Coast family icon. This protein-packed, fresh burger captures classic California fast food at its finest.
Prepare the special sauce: Combine mayonnaise, ketchup, sweet pickle relish, white vinegar, sugar, and a pinch of black pepper in a small bowl. Mix thoroughly and chill in the refrigerator for at least 20 minutes for flavors to meld.
Slice the tomato and onion thinly; rinse, dry, and prepare crisp whole lettuce leaves. Set aside on paper towels to prevent sogginess.
Divide the ground beef into four even portions (6 oz each) and shape into patties just slightly wider than your buns. Season both sides with kosher salt and black pepper.
Preheat a skillet (cast iron is ideal) over medium-high heat. Lightly butter the cut sides of buns. Toast buns in the skillet, pressing down for even color, until golden; set aside, keeping them warm.
Increase heat to high. Add beef patties to the skillet. Cook for 2–3 minutes per side without pressing down, flipping once you see juices pooling on the surface. Add a cheese slice on top in the last minute. If you prefer, grill onions simultaneously until soft and caramelized, about 5 minutes, stirring occasionally.
Assemble burgers: Spread special sauce generously on both top and bottom buns. Layer lettuce on the bun base, followed by tomato slices, then the hot cheesy beef patty. Top with grilled onions, if using, and finish with the top bun.
Serve immediately while the cheese is gooey and the lettuce crisp.