Heat a large heavy-bottomed pan, then seal the lamb or mutton in olive oil. Add the onion and cook until soft. Pour in about half of the stock, enough to cover the meat, and bring it to the boil. If you are using mutton, add a dash of Worcestershire sauce to taste. Cover the pan with a lid and leave the meat to simmer for 1 hour.
At this point, add salt and pepper. If you have used neck of lamb, let the mixture cool, skim off the fat, take all the meat off the bone and stir it back into the pan. Place it back on the heat.
Add the chopped potatoes, cook for 5 minutes, and then add the carrots, turnip and the remaining stock. Bring to the boil and simmer for 10 minutes. Taste the stew to check the seasoning, then make sure the vegetables are covered with the liquid. Stir in the sliced leek and cook for 5 to 8 minutes, or until everything is cooked.
Lobsgows is best eaten the next day when the stew has thickened and flavours have intensified. Serve with homemade buttered bread.