Place diced potatoes in a medium saucepan and cover with water. Place over high heat and bring to the boil. Boil for 5 minutes, or until potatoes are tender. Drain.
Place a large frying pan over medium heat. Add olive oil, then onion. Toss lightly to coat. Saute onion for 5 minutes, then add cooked potatoes. Saute mixture an additional 10 minutes, until onions are caramelized and potatoes are lightly browned.
Place potatoes and onions in a large bowl. Add hard-boiled eggs, olives and sultanas. Mix well. Add salt and pepper to taste.
Roll out pastry and cut 32 8cm circles. Place a heaping tablespoon of filling on one half of each circle. Fold pastry in half and crimp edges with a fork. Place empanadas on a baking tray and brush each one with beaten egg.
Bake in preheated oven for 25 to 30 minutes, until golden brown. Serve hot.
Alternatively, you can deep fry the empanadas in oil that's reached 180 degrees C. You can also make them into full circles rather than half circles, making 16 larger empanadas.