Combine ground beef, egg, breadcrumbs, onion and 2 tablespoons ketchup.
Shape into 4 mini burger patties. Heat oil in frying pan and cook patties until browned on each side and cooked through.
Place cheese on top of patties during last few minutes of cooking to melt.
Split rolls in half and top with patties, tomato, lettuce and ketchup to make mini burgers.