Makes 16 individual pastries.
For the dough, mix the flour and salt together. Add the oil and stir by hand or in the bowl of a stand mixer with the paddle attachment, to coat the flour with the oil (there will be no visible difference). Add the butter and cut in until no large pieces of butter are visible. Whisk the milk and egg together and add this all at once to the flour, mixing until the dough comes together. Shape the dough into 2 cylinders, wrap each in plastic and chill for at least 2 hours before rolling.
For the filling heat a large sauté pan with the oil over medium heat. Add the onion and cook for 2-3 minutes to soften it and then add the beef, cumin, paprika, chili powder and oregano, stirring often and cooking the beef all the way through. Remove the pan from the heat and cool the meat completely (this can be made ahead and chilled).
After cooling, stir the green onions and olives into the meat. Use a box grater to grate in the hard boiled eggs. Season to taste.
Preheat the oven to 375ºF (190ºC) and line 2 baking trays with parchment paper.
Unwrap the dough cylinders and slice each into 8 pieces. Roll out each piece to a circle just under ¼ -inch (6 mm) thick and use a 5 or 6-inch round cutter to trim the edges. Brush the outside edge of each circle with a little of the egg white and spoon meat filling in the centre of each. Fold the pastry over the meat filling, and then either use a fork to press and seal the edges, or twist the dough, overlapping it as you fold, to create a seal. Place the empanadas on the baking tray. Whisk the egg yolk with a tablespoon of water and brush the surface of each. Bake the empanadas for about 30 minutes, until a rich golden brown.
The empanadas are best served warm.