Coat the beef in flour and set aside. Place the diced potatoes and finely chopped onion into the bottom of a slow cooker.
To make the sauce, whisk together the massaman curry paste, beef stock, coconut milk, fish sauce, lime juice, and brown sugar until well combined.
Pour the massaman sauce over the potatoes and onions, then put in the floured beef. What's great is there is no need to stir.
Cover and cook on low for a minimum of 8 hours.
Sprinkle with chopped coriander and serve with steamed rice. If you'd like an extra zing, squeeze some lime juice over the top.