1. Heat the oil in a medium non-stick frying pan, then add the chorizo. Sizzle for 5 minutes until the chorizo is starting to crisp and has released its red oil. Scoop the sausage onto a plate. Now stir the leeks into the fat and juices and soften for 5 minutes.
2. Stir in the chorizo, potatoes and most of the rosemary, then lower the heat and pour in the eggs. Cook gently for about 8 minutes, gently shaking the pan now and again, until the tortilla is set almost all the way through. Heat the grill to high while you wait.
3. Slice the cheese into rounds, spread over the tortilla and season with black pepper and a little more rosemary. Grill for 2 minutes or until the egg is set and golden and the cheese is melty.