For a deeper flavor, marinate the lamb in the stout for at least 2 hours or overnight before cooking. If you don't have pearl onions, you can substitute with 1 large onion, roughly chopped. The stew can be made a day ahead. The flavors will meld together even more, making it even tastier the next day. Reheat gently before serving. If you want to add more vegetables, consider turnips or parsnips for an authentic flavor.