Portion ground beef into 12 balls weighing 2 ounces each (about the size of a golf ball).
Cut rolls horizontally to keep tops and bottoms intact, respectively. Spread each cut side with butter. Preheat the griddle surface to medium-high. Toast buns until golden; set aside.
Add canola oil and diced onion to a hot griddle. Spread to an even layer; sear for 3 minutes. Place beef balls over cooked onions and with a solid metal spatula, smash balls into patties. Season with salt and pepper. After 1 to 2 minutes burgers should have a good sear on the first side. Flip burgers and arrange them closely in 3 rows of 4 (to match bun set dimensions). Place cheese slices evenly on top of the group of patties.
Spread mayo, mustard, and pickles across the bottom buns. Top with slider patties, making sure to scoop up any onions with burgers. Don’t worry if it’s not pretty—just do your best to keep the patties whole. Top with shredded lettuce and top buns. Serve together for folks to grab their own or cut into individual sliders.
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