Peel and wash the potatoes and cut them into thin slices. Peel onions and cut into fine slices. Fry potatoes in hot oil in portions for about 15 minutes, add onions at the end and cook for a few minutes. Season with salt and pepper. Put everything together in a rectangular form (approx. 30x20 cm). Whisk the eggs and season with salt and pepper.
Pour the eggs over the potatoes. Let it simmer in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 15 minutes. Take out of the oven, let it cool down and cut into approx. 30 cubes. Peel the Parmesan cheese with a peeler. Garnish the tortilla cubes with either olive or a slice of ham and Parmesan cheese cut in half lengthwise, and stick them firmly with small wooden sticks.
With 10 people: