Mix the burger ingredients together in a bowl then divide the mixture into 4 balls and flatten into patties, or use our burger press. Put them in the fridge on a plate covered with clingfilm and leave for at least an hour to firm up.
Heat the grill or griddle to medium heat. Rub a little oil over the burgers and grill or fry them on each side for 6-7 minutes or until cooked through.
Meanwhile, cut the bread buns in half and toast the insides. Spread a little mayonnaise on the bottom halves and add lettuce and slice of tomato to each.
Place the cooked burgers on the tomato and top with onion, a dollop of relish and the bun tops.