1Place a Dutch oven or large saucepan on medium-low heat. Add butter, onion, leeks, and a generous sprinkle of salt. Return the lid and cook gently for about 10 minutes until really soft and slightly golden and creamy.
2Stir in the rosemary and garlic and let this cook for a couple of minutes.
3Pour in the chicken stock.
4Grate the zest of one lemon into the soup.
5Add in the chicken breasts, making sure they are submerged in the stock. Return the lid and set your timer for 5 minutes, ensuring the stock is simmering gently.
6After 5 minutes, add in the pasta shells, return the lid, and set the timer for 4 minutes.
7After 4 minutes, remove the chicken breasts into a bowl to cool slightly before shredding with two forks.
8Add the lemon juice to the soup and remove the soup from the heat when the pasta is al dente.
9Return the shredded chicken to the soup.
10Add the parmesan, double cream, and a generous crack of black pepper.
11Stir to combine all ingredients.
12Serve with more parmesan, black pepper, and a drizzle of olive oil.