To make the salsa, combine all the ingredients in a bowl. Season with freshly ground black pepper and set aside.
Cook the potatoes in a large saucepan of boiling water for 12–15 minutes or until just tender. Drain and cool slightly before cutting in half.
Heat the olive oil in a 22cm flameproof non-stick frying pan over medium heat. Add the onion and cook, stirring occasionally, for 3 minutes or until starting to soften. Add the potato and cook, tossing occasionally, for 5 minutes or until golden and crisp.
While the potato is cooking, whisk together the eggs and milk in a jug. Season with freshly ground black pepper.
Reduce the heat to low. Pour the egg mixture over the potato and onion and cook, gently stirring occasionally, for 1 minute. Sprinkle over the cheddar and cook, untouched, for 8–10 minutes or until the egg has set firmly in the centre.
Meanwhile, preheat the oven grill to high.
Transfer the pan to the grill and cook for a further 2 minutes or until the top is set and golden. Remove and leave to rest for 5 minutes before turning out and slicing.
Serve the tortilla warm topped with yoghurt, salsa and a sprinkling of basil leaves, if desired.