Cook the bacon in a skillet over medium heat for 7 to 9 minutes until browned, then spoon off all but 1 tablespoon fat.
Add the onion and cook for 6 minutes, then stir in the coffee, brown sugar and vinegar.
Simmer for 6 to 8 minutes until the bacon jam is dark, sticky and almost dry, then keep it warm.
Heat a steel griddle or cast-iron skillet over high heat and set the cold beef balls on the bare surface.
Immediately press each firmly through parchment into a thin patty, season and cook untouched for 90 seconds; do not re-smash.
Scrape, flip and cook for 60 to 90 seconds until the beef reaches 160°F, adding half a cheddar slice to each for the last 30 seconds.
Toast the slider buns for 1 minute and fill each with arugula, one cheesy patty and 1 tablespoon bacon jam.
