To make the Big Mac sauce, combine the first 10 ingredients in a bowl. Use a small whisk to blend fully.
Saute onion slices in a frying pan with butter. Set it aside with other prepared burger toppings.
Heat up your grill to approximately 350°. Set a cast iron pan in the grill to preheat.
While the cast iron is heating up, prepare the slider patties. They should be quite thin, so we recommend forming them on a small square of parchment paper that can be used to help transfer the burgers to the pan. Simply salt the top of the patty.
Spray the cast iron pan with non-stick cooking spray and flip the burger patties onto the pan. Sprinkle salt on the now exposed unseasoned side. Close the grill for about 90 seconds.
Flip the burgers over using a spatula and close the grill for about 45 seconds.
Add American cheese slices to the burgers. Close the grill for another 45 seconds.
Remove burgers from the grill.
Place pretzel bun rolls on the grill grates for about 30 seconds to lightly toast.
To assemble the big mac sliders, we recommend building from the bottom bun, a layer of big mac sauce, one cheeseburger patty, a pickle slice, sauteed onions, lettuce, a second cheeseburger patty, more big mac sauce, pickle, onion, lettuce and top bun.