Place the curry paste, garlic and ginger in a mini blender. Reserve some coriander sprigs for garnish and add the rest to the blener along with four tablespoons of the coconut milk. Blend to a smooth paste.
Place the beef in a bowl and stir in two tablespoons of the curry paste mixture; cover with clingfilm and leave to marinate in the fridge for at least 2 hours, if possible, or up to 24 hourse is great. Place the rest of the curry paste mixture in a small bowl and cover with clingfilm, then chill until needed.
Preheat the oven to Gas Mark 2, 150°C (300°F). Heat a wok or frying pan until smoking hot. Add the curry paste mixture and stir-fry for 1 minute, then add a third of the beef and stir-fry for another 4-6 mintues until well sealed and golden brown. Tip into a casserole dish with a lid and set aside. Then repeat with the remaining curry paste and meat.
Pour the rest of the coconut milk, and the stock into the wok or frying pan and bring to the boil. Then pour it over the meat in the casserole dish, cover and place in the oven for 1 and a half hours until the beef is meltingly tender but still holding its shape. Remove from the oven and stir in the lime juice and the fish sauce. \serve in deep bowls with steamed rice and a sprinkling of the reserved scallions and coriander sprigs.