1.Heat butter and half of the oil in large saucepan. Stir chopped onion and garlic over heat until onion softens. Add combined spices. Continue stirring until fragrant. Add potato, stock and the water; bring to a boil. Reduce heat and simmer with lid on until potato is just tender. Cool 10 minutes.
2.Meanwhile, heat remaining oil in small frying pan. Shallow fry the sliced onion, stirring, until browned and crisp. Drain on absorbent paper.
3.In batches, blend or process soup until smooth. Divide among serving bowls; top with yoghurt and fried onion.