Heat a steel griddle or cast-iron skillet over high heat and toast the buttered slider buns for 1 minute.
Scatter the onion in 6 small piles on the hot surface and place one cold beef ball over each pile.
Immediately press each ball firmly through parchment into a thin patty, then season with salt and pepper.
Cook untouched for 90 seconds until the edges brown; do not re-smash the patties.
Scrape, flip onion-side up and cook for 60 to 90 seconds until the beef reaches 160°F.
Add half a cheese slice to each patty for the final 30 seconds.
Spread mustard on the buns and assemble each slider with one patty and 3 pickle chips.