Combine turkey, herbs, onion and sherry. Mix lightly then form into patties. Set aside, covered for about 20 minutes, Heat a skillet and add the butter and olive oil. Add the patties and cook until browned on one side, then turn and brown on the other. Just before done, top with cheddar and cover pan. Continue to cook until the cheese has melted. Remove the burgers from the pan and add the Worcestershire sauce, and whisk thoroughly. Use as sauce for the burgers. Good with potato bread buns.