Heat the oil in a large saucepan and fry the onion until soft. Add the chopped carrots, potatoes, tomatoes, water, stock cube and seasoning. Bring to a boil and allow to simmer until the vegetables are cooked—about 25 minutes. In the meantime, mix the cornstarch with a little water to make a smooth, liquid paste. Put the yogurt into a small pan and mix in the cornstarch. Bring to a boil, stir constantly and allow to cook over a low heat for 2-3 minutes. Add the yogurt to the vegetables and mix well. Process in batches and then reheat just before serving. Garnish with chopped parsley and lemon rind.