Shape the beef patties and season with salt and pepper. Fry in a hot pan with a bit of oil for 3 minutes on each side. Place slices of Castello® Extra Matured Cheddar on each patty, after they have been flipped the first time. Heat the slider buns. Mix mayonnaise and mustard and spread on the bottom half of the slider buns. Add lettuce, tomato and pickle slices, beef patty with cheddar and onion slices – finish with ketchup and the top half of the slider bun. Serve as a snack or with fries and a milkshake.