Cook mushrooms and onions in 2 tb dressing over med-high heat. Add flour and seasonings; gradually add milk, stirring until thickened. Stir in 1 cup dressing, turkey, vegetables and 1 cup cheese; cook 5 minutes. spoon mixture into 1 1/2 quart cassrole. Stir 1/2 cup dressing into potatoes. Top casserole with potato mixture; broil until golden. Sprinkle with remaining cheese; broil until cheese begins to melt.