There are two important factors to the perfect tortilla. First, the right-sized pan is crucial. You need a small frying pan, about 15 cm/6 inches in diameter. As these Spanish potato and onion omelettes are perfect for everything from breakfast to picnics, it’s well worth the investment. The second lies in the caramelisation of the onions… low and slow, so that golden hue is teased out, adding immeasurable depth to the flavour. You want a gently oozing centre, too.