Mix the beef, cracker crumbs, cold water, egg, salt and pepper with a fork just until evenly combined.
Divide the mixture into 6 portions, press each between parchment into a compact 1/3-inch-thick patty and chill for 10 minutes.
Pour the oil into a deep cast-iron skillet to about 1/4 inch and heat it to 350°F.
Lower 3 patties into the oil with a slotted spatula and fry for 3 to 4 minutes per side until dark golden and 160°F in the center.
Drain the patties on a wire rack rather than paper towels so their thin crust stays crisp, then repeat with the remaining patties.
Warm the buns in a dry skillet for 30 seconds without browning them hard.
Place one patty on each bun and top with 1 tablespoon mustard, chopped onion and 3 pickle chips.