Melt the lard in a large skillet over medium-high heat and add the white onion, lightly frying it until translucent. Add the spices and then the beef, stir frying for 1 to 2 minutes.
Add the reserved potato water and then the diced potato, spring onion and salt to taste. Remove from the heat and cool. Stir in the egg.
Preheat the oven to 400 degrees F. If you make an empanada dough, roll it out to just under 1/4-inch and cut out circles of about 4 inches in diameter.
Holding the circle of dough in the palm of your hand, wet the outer rim of one half of the circle with a big of beaten egg– this will help you seal the empanada. Place a heaping spoonful of the mixture in the middle and join the two halves of the circle.
Going from left to right, twist and fold the edge of the dough over onto itself to form the traditional repulgue (or you could complete the seal by pressing down with the point of a fork).
Place on a baking sheet. Repeat with the remaining dough and filling. Brush the formed empanadas with beaten egg yolk. Bake for about 15 minutes, or until golden brown.