Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the ground beef, salt, and black pepper. Mix until just combined; do not overmix.
Divide the beef mixture into 12 equal portions and shape each portion into a small patty, approximately the size of the slider buns.
Heat a large skillet over medium-high heat. Once hot, cook the patties for about 3-4 minutes on each side, or until they reach your desired doneness.
One minute before the patties are done cooking, place a slice of cheddar cheese on each patty to melt.
While the patties are cooking, you can toast the slider buns on the baking sheet in the oven for about 5 minutes if you prefer them warm and slightly crispy.
Assemble the sliders by placing a cooked patty on the bottom half of each bun. Top with diced onions and pickles, and add ketchup and mustard to taste before placing the top bun on.
Serve immediately and enjoy these tasty cheeseburger sliders!