Preheat the oven to 200C

Mix together the onion, beef, egg, breadcrumbs, ketchup, Worcestershire sauce, mustard, salt and oregano in a bowl

Separate into 6 even portions and mould into each portion in to a burger

Heat a frying pan on a medium heat and sear the burgers on each side for 2 minutes on each side (this may need to be done in batches)

Then cook the burgers in the oven for 6-8 minutes or until cooked throughout. For the last couple of minutes, add the cheese on top of the burger and let it melt.

Stack the burger. First, the base of the burger bun, then lettuce, the burger/cheese and the burger top.