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.
Bake in hot oven (425°F.) until crust is golden, about 40 minutes. Remove from oven and let stand a few minutes. Run spatula under outer edge of pie. Invert serving plate over top of pie pan. Invert both and lift off pie pan.
Dot top of meat with remaining ketchup and sprinkle with coarsely snipped (with scissors) parsley. If parsley is not available, add color by scattering shreds of yellow cheese over ketchup-dotted meat pie.