Heat the oil in a large casserole over a high heat. Season the meat with plenty of salt and pepper, then add the lamb to the casserole and sear on all sides until it has developed a rich brown crust. You will probably need to do this in two batches. Remove from the casserole and add the onion. Fry until lightly browned, then stir in the garlic, coriander seeds, thyme and tomato purée. Continue to cook for a few minutes, stirring constantly, and then pour in 1 litre/1¾ pint water.
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.