Heat the olive oil in a good non-stick pan over medium heat. Add the onion and cook for around 7-8 minutes until softened and browned.
Add the sliced potatoes and cook for a further 3–4 minutes, stirring frequently.
Add the salt and water, cover with a lid, and reduce the heat to low-medium. Cook for around 15 minutes, stirring occasionally to prevent sticking. The potatoes should be softened but don't need to be fully cooked.
Meanwhile, add all of the eggy mixture ingredients to a blender and blend until completely smooth.
Preheat the oven to 200°C and line a round cake tin or baking tin with parchment paper.
Combine the cooked potatoes and onions with the eggy mixture in a bowl, and transfer to the prepared tin.
Bake for 10 minutes, then remove and lightly brush the top with olive oil. Return to the oven for a further 10 minutes until lightly golden.
Remove from the oven and allow to cool before lifting out of the tin. This helps prevent the tortilla sticking to the parchment paper.
Top with fresh chives and serve warm or cold.