Heat olive oil in a small frying pan.

Gently fry sliced onion over medium heat until soft.

Transfer softened onions to a bowl.

Discard remaining oil from the pan.

Heat vegetable oil in a deep, heavy-based frying pan.

Test oil temperature with a breadcrumb (it should sizzle and turn golden brown).

Alternatively, use an electric deep-fat fryer at 170C/325F (CAUTION: Hot oil can be dangerous. Do not leave unattended.).

Add thinly sliced potatoes to the hot oil and cook until soft (test with a fork). Don't brown them.

Remove potatoes from oil and drain on kitchen paper.

Once cooled, add potatoes to the bowl with the onions.

Add beaten eggs to the potatoes and onions.

Season with salt.

Gently mix with a fork to break up potato slices slightly.

Pour the potato mixture into the pan previously used for onions.

Flatten the mixture.

Cook over low heat for 5-10 minutes, until the egg sets on top and the bottom starts to color.

Slide the tortilla out of the pan onto a plate, cooked side down.

Place the pan over the uncooked side of the tortilla, then swiftly turn the plate and pan over (holding tightly).

Place the pan back on the heat.

Cook for a further 5 minutes until the underside is light brown.

Remove from heat.

Serve hot or cold.

Cut into slices and serve as part of a tapas selection.