Preheat the Cuisinart® Griddler® Jr. in the closed position to high.
Add the meat, onion, salt and pepper to a medium bowl. Mix gently with hands, being sure not to overmix. Divide the meat into 12 individual rounds (about ¾ ounce each). Form into patties.
When the Griddler® is hot, arrange the mini burgers on the bottom grill. Close, applying light pressure to handle for about 10 seconds. Grill for 1 to 1½ minutes. Remove the burgers; place each burger on the bottom of each miniature bun. Top each burger with the shredded cheese; place the top bun on top. Return assembled cheeseburgers to the bottom grill place. Close, applying light pressure for about 20 to 30 seconds, until the cheese has melted. Serve immediately.