Heat 2 tablespooons of the oil in a large casserole dish and brown the beef on all sides. Set aside, add 1 1/4 cups cold water to the dih and scrape the bottom of the pan with a wooden spoon. Retain these juices.
Heat the remaining oil and cook the onion and chili peppers for 5 mins until soft.
Add the curry paste and turn up the heat for 2 mins or until the onion is going a light brown in colour.
Return the beef to the pan along with the beef juices, coconut milk and broth.
Cook, uncovered, over a very gentle heat for 1 1/2 hours so that it is barely simmering and the beef becomes tender.
If the coconut milk boils it will split so keep an eye on it.
Once the beef is quite tender, add the sweet potatoes and cook for 20 mins or until they are soft.
Season with the fish sauce and soy sauce.
Sprinkle with cashew nuts and cilantro and serve with a bowl of rice on the side.