MIX turkey, parsley, Worcestershire sauce and onion salt in large bowl. Shape into 4 (4-inch) patties; set aside.
MELT butter in large skillet on medium heat. Add mushrooms; cook and stir 2 to 3 minutes or until tender. Remove mushrooms from skillet; cover to keep warm. Add patties to skillet; cook on medium heat 12 to 14 minutes or until cooked through, turning occasionally. Serve in buns; top with mushrooms.