Peel and finely slice the potatoes and onion. Beat the eggs in a bowl. Slice the capsicums lengthways and remove the seeds.
Heat a generous amount of olive oil in a medium frying pan. Add potatoes, onion and salt to taste and fry until tender and golden. Drain with a slotted spoon and add to the beaten eggs. Mix well.
Drain the old oil from the pan and wipe clean with a piece of kitchen paper. Heat again with a good drizzle of new oil. Pour the entire mixture back into the pan. Cook over medium to medium-low heat until the bottom is set.
When the bottom is set, carefully place a plate over the top of the pan. Invert the tortilla onto the plate, then slide back into the pan to cook the other side. Once set, slide the tortilla out of the pan onto a serving plate.
Add a little more oil to the frying pan and fry the green capsicum slices with a little salt until tender. Arrange over the top of the tortilla. Serve immediately or at room temperature.