For stuffing, melt butter in a large skillet over low heat. Combine stuffing, parsley, lemon pepper, tarragon, salt, egg, sherry and 2 tablespoons lemon juice in a large bowl. Add cooked mixture and mix gently. Cut slits about 2 inches deep between ribs of roast, without cutting through opposite side. Pack stuffing into slits.
Place roast on rack in shallow roasting pan.
Insert meat thermometer in thickest part of roast. Place a piece of aluminum foil loosely over top of roast just to cover stuffing, making a slit in foil to fit around meat thermometer. Let roast stand for 10 minutes before carving. Garnish with fresh tarragon sprigs and lemon slices, if desired.