Put the potatoes in a large pan of cold water, bring to the boil and cook for 10-12 minutes until just tender.
Heat the oil in a non-stick ovenproof frying pan and cook the onion for 8 minutes, until beginning to soften and brown. Add the cooked potatoes. Pat dry the courgettes with kitchen paper and add to the pan with the thyme leaves.
In a bowl, beat together the eggs, ricotta and lemon zest, then season with freshly ground black pepper.
Sprinkle the Cheddar over the potatoes and onion and spread out evenly in the pan. Pour the egg mixture over the potato and onion mixture and cook over a medium heat for 15 minutes.
Preheat the grill to medium. Put the tortilla under the grill and continue to cook for 5-8 minutes until the top is just cooked.
Carefully turn out onto a board and allow to cool before cutting into wedges.