1Carefully assemble the processor with the grey spindle and green S-blade™ in the bowl. Add the beef and pulse until evenly chopped. Add the salt and pepper and process until the beef is ground to your liking.
2Divide the beef into 8 even portions and shape each portion into a 1½-inch round patty.
3In the clean processor bowl with the grey spindle and green S-blade™, separate the onion layers and process until finely chopped. Remove and set aside.
4Assemble the processor with the grey spindle and orange adjustable slicer set on 3 in the bowl and slice the lettuce.
5Place a large frying pan over high heat. Add the oil, when hot add the patties in batches and press to smash. Cook until golden brown. Flip the patties over and place a slice of cheese on top of each patty. Continue cooking until the patties are cooked to your liking. Make a stack of 2 patties. Remove the patties from the pan and wipe the pan clean.
6Split the buns in half and spread with butter. Place the pan over medium heat and toast the buns, cut side down, in batches, until golden brown.
7Spread a tablespoon of the mayonnaise over the base of each bun. Top with the lettuce, patties and onion. Spread the ketchup and mustard on the top of each bun. Sandwich the burger with the top of the bun and serve.