Preheat the oven to grill.
Preheat a BBQ or frying pan to a medium heat. Brush the patties with a little oil and cook for a few minutes until browned on one side. Turn the patties over, reduce the heat to medium-low and cook for another 8-10 minutes until cooked through.
Transfer the patties to a roasting dish or oven tray and lay a slice or two of cheese on top of each one. Cook under the grill for a minute or so until the cheese starts to melt over the patties.
To make the secret sauce, combine all the ingredients and season as required.
Place a patty onto the bottom bun, add the lettuce leaves, slices of red onion and top with a dollop of sauce.
