Return the lamb to the casserole and bring to the boil. Turn down the heat, cover and leave to simmer for 1 hour. Add the celery, carrots, potatoes and cabbage, cover and cook for another hour.
While the vegetables are cooking, make the dumplings. Mix the cornmeal, flour, baking powder and thyme, if using, in a bowl and season with plenty of salt and pepper. Add the butter and rub it in until you have a mixture resembling fine breadcrumbs. Gradually add the milk and work until you have a dough. Divide the dough into six dumplings.
Add the sweetcorn to the casserole and push down into the liquid. Place the dumplings on top, cover and cook for around 20 minutes, until the dumplings are well risen and cooked through.