Pour 2 litres of water into a large pan with 2 teaspoons of sea salt, then bring to the boil over a high heat. Peel and add the whole onion, along with the lamb. Bring back to the boil, skimming away any scum from the surface. Simmer on a medium heat for 10 to 15 minutes, or until the lamb is cooked through. Using a slotted spoon, remove the lamb to a plate and leave until cool enough to handle. While it’s cooling, peel the swede, chop into 1cm chunks, and add to the pan to get a headstart. Peel the carrots and parsnips, slice at a slight angle 1cm thick, and drop them into the pan. Now peel the potatoes and cut into 4cm chunks. Strip all the lamb meat from the bone, and return the meat to the pan with the potatoes. Bring back to the boil, then simmer it all for 15 to 20 minutes, or until almost tender, while you wash the leeks and cut them into 1cm-thick slices. Now you can eat this straight away if you want to – simply stir the leeks into the pan, bring to the boil again, then simmer for 10 minutes with the lid on, or until tender. Taste and season to perfection. For even tastier results, let it chill overnight. In which case, simply stir in the raw leeks, cover, and pop into the fridge, where it will keep for up to 3 days. When you’re ready to serve, gently simmer the cawl until warm through, then season. Either way, ladle into serving bowls and serve with lots of black pepper, a wedge of mature Caerphilly cheese and a slice of good bread and butter for dunking.