Rehydrate the onions by covering them with 1/2 cup lukewarm water in a small bowl. Set aside while you prep the patties.
On parchment paper lined quarter sheet tray (or metal 13×9 baking dish) loosely break up the beef into an even layer. Season generously with kosher salt. Gently press the beef down into a single thin layer, pushing it into the corners of the tray. Finally top the beef with more parchment and use another tray of baking dish to press down on the beef. Remove the top tray and paper. Use a knife to cut the beef into 12 squares then dock the squares with the thick end of a skewer – this helps the bun steam and keeps the patties from shrinking up.
Drain the onions. Heat a flat top or large skillet to medium high heat. Sprinkle on the onions and cook for 1 minute. Add the beef patties, giving some space between them. Top each patty with a bottom bun. Cook the patties for 3 minutes total time. Flip the patties and remove from the grill, onions up. Top each patty with a dill pickle slice and top with the other bun half.