Combine beef, onion, salt, mustard, pepper, bread crumbs, egg, milk and ⅓ c. ketchup. Place mixture in greased 8" round cake pan. Pat meat out to within ½" of sides of pan.
Roll pastry out to a scant 10" circle. Place it over meat, tucking it down around meat and pressing it against bottom of pan. Prick pastry all over with a four-tined fork.