Heat two tablespoons of the oil in a pan and gently cook the onion until soft but not brown.
Add the mushrooms and cook until just softened.
Transfer the mixture to a large bowl and gently mix in the herbs, minced turkey and beaten egg.
Season the mixture with salt and pepper and shape into burgers.
Heat the remaining oil in a griddle pan and cook the burgers for about 3 minutes on each side or until browned and cooked through.
Place the burgers in the buns with the lettuce and top each one with a tablespoon of cranberry sauce.