Preheat the oven to 350 degrees. In a large mixing bowl, combine mushrooms, turkey, onion, seasoning, breadcrumbs, and egg; mix until evenly combined. Form into lightly greased loaf pan. In a small bowl mix together mustard, sugar, and ketchup. Pour sauce over top of meatloaf, spreading until even. Cover with foil. Bake for 60 minutes, or until cooked through.