Divide the ground beef into eight 2-ounce portions. Shape into patties.
Season both sides with salt and pepper. Spread mustard on one side.
Melt the beef tallow in a pot or in the microwave.
Heat a griddle to medium high. Brush the griddle with the melted beef tallow.
Add the patties, mustard side down. Add a handful of sliced onions on top of each patty. Cook, until the patties develop a crust. Flip.
Top each patty with American cheese, two pickles and a top bun.
Continue cooking for a few minutes, until the meat is cooked through and the cheese is melted. Transfer to a platter with the bottom buns.
