1. In a small pot over medium-high heat add diced potato and pour in enough cold water to cover by 1-in (2.5 cm). Bring to a boil and cook until just barely tender. Drain and let dry.
2. In a saute pan over medium-high heat, warm olive oil. Add onion and garlic and saute until softened, about 5 minutes. Stir in cooked potatoes, pimentos, cumin and oregano and cook for 1 minute more. Reduce heat to medium-low and add tomato sauce. Simmer until mixture thickens, about 2 to 3 minutes.
3. Remove from heat and stir in flaked mackerel, raisins and capers. Let mixture cool to room temperature.
4. Spoon about 3 tbsp (45 ml) of the mackerel mixture into the center of a dough circle. Flatten the filling slightly, keeping it away from the edges. Brush the edges with a bit of water and fold one half of the dough over the other. Press gently to seal, then crimp the edges with your fingers or the back of a fork. Repeat with the remaining dough circles and filling.
5. Transfer the filled empanadas to a parchment‑lined baking sheet and brush with beaten egg. Empanadas can be made ahead and frozen for up to 3 months.
6. To cook empanadas from fresh, bake at 400°F (200°C) for 20 to 30 minutes, until golden and flaky, or air‑fry at 375°F (190°C) for 12 to 15 minutes. To cook from frozen, do not thaw. Add 8 to 10 minutes to the cook time.

To make the empanada discs:

1. In a mixing bowl, combine flour and salt and mix well. Add shortening and work it in until all the pieces are coated. Pour in water and cider vinegar and mix until a rough ball of dough forms.
2. Turn dough onto a lightly floured surface and knead a few times until the flour is fully incorporated. If it feels a bit sticky, sprinkle a little more flour over top and knead a few more times.
3. Wrap and chill dough in the refrigerator for at least 30 minutes.
4. Divide the rested dough into 16 even pieces and roll each into a ball. On a lightly floured surface, roll each into a disc, about ⅛-in (0.3 cm) thick, trimming the edges a bit if needed.
5. Empanada dough rounds can be prepared in advance and frozen, for up to 3 months, stacked with parchment between each layer.