Peel and very finely slice the onion and potatoes, then toss and mix together. Pour 1cm of olive oil into a 24cm non-stick ovenproof frying pan on a medium heat. After 1 minute, add the potato and onion mix (don’t season at this stage, as it will draw the moisture out of the potatoes) and fry gently for 25 minutes, or until tender but mostly without colour, lightly tossing occasionally. Drain the potatoes and onions and leave to cool slightly. Beat the eggs in a bowl, add the cooked potato and onion, then gently fold together and leave for 5 minutes – the potato will start to absorb the egg, and you should see little bubbles surrounding the potato and onion. Drain and finely slice the peppers, then fold into the egg mixture and season to perfection with sea salt and black pepper. Preheat the oven to 160°C/325°F/gas 3.
Quickly wipe out the pan, drizzle in a little olive oil, then place on a low heat. Pour the egg mixture straight into the pan and spread out. Leave for 5 minutes, then bake for 10 minutes. When the time’s up, remove from the oven, leave to rest in the pan for 5 minutes, then run a knife around the edge to loosen. Quickly and confidently turn it out, using a plate or flat lid that’s bigger than the pan, and serve sprinkled with a little salt and a pinch of watercress. Delicious hot or cold.