In a mixing bowl, combine mayonnaise, ketchup, mustard, apple cider vinegar, garlic powder, onion powder, and a pinch of salt and pepper. Mix well and set aside to create the special sauce.
Preheat the skillet or griddle over medium-high heat.
Divide the ground beef into four equal portions, shaping them into loose balls.
Once the skillet is hot, place each meatball into the skillet and flatten it with a spatula to create a patty. Season with salt and pepper.
Cook for about 3-4 minutes or until the bottom is caramelized and browned. Flip the patties and immediately place a slice of cheese on each one. Cook for an additional 2-3 minutes until the cheese is melted.
In the final minute of cooking, add diced onions to the skillet to sauté slightly.
To assemble the burgers, place a lettuce leaf down, add a cheese-topped patty, a spoonful of special sauce, sliced pickles, and shredded lettuce. Top with another lettuce leaf.
Serve immediately and enjoy your keto Big Mac Smash Burgers.