Get a large pan out and throw in the coconut milk, fish sauce, salt, lemongrass, cardamon, cinnamon and ginger along with the beef. Slowly bring to a simmer, partially cover and simmer for 2 hours until beef is very tender, stirring gently occasionally.
Get the tamarind and put it in 475ml of boiling water and let it infuse for 15 minutes, then pulse in the blender for a few whizzes. Then strain through a sieve, pushing as much through as you can.
Take the whole spices out of the beef mixture, and add the curry paste, chilli, potatoes and shallots, then add the tamarind water. Cover and simmer until the potatoes are cooked.