2.Heat butter and oil in large frying pan and cook garlic, cumin, oregano and potato, stirring occasionally for about 10 minutes or until potato is browned. Add tomato and olives, cook, stirring, about 10 minutes or until liquid has evaporated. Transfer to large bowl, stir in cheese.
3.Meanwhile, using sharp knife, make a small horizontal cut in each chilli 1cm below stem, then make lengthways slit in chilli, starting from horizontal cut and ending 1cm from tip, taking care not to cut all the way through, discard membrane and seeds. Carefully divide filling among chillis, securing each closed with a toothpick.
4.Make tomato sauce. Heat oil in large saucepan and cook onion, garlic, cumin and oregano, stirring, until onion softens. Add undrained tomatoes and the water, bring to the boil. Reduce heat; simmer, uncovered, 10 minutes.
5.Place chillies on tomato sauce in dish, cover and cook in oven 40 minutes or until chillis are tender. Serve with tomato sauce.