Tip the beef mince, onion and egg into a bowl, then mix.
Carefully roll out into balls - roughly the size of a tennis ball.
Gently flatten in your palms, until about 3cm in thickness.
Pop burgers on a plate, cover with cling film and place in fridge to firm for 30mins.
Pan fry in oil 5 mins on each side (your burgers will be pink in the middle).
Cook for longer if desired.
Serve on bun of choice with cheese (if using).