Form the beef mince into four patties but don't pack them too tight or they won't be juicy. Season both sides well with salt and pepper.
Heat a frying pan over medium high heat and fry the burgers, 6 minutes for medium-rare, 8 minutes for medium or 10 minutes for well done. Flip them once during the cooking. If using cheese, lay the slices on top of the burgers for the last minute of cooking and let melt.
Spread the buns with tomato sauce and mustard, add the burgers and top with tomato, onion and lettuce. Enjoy!