Preheat the oven to 350 degrees F.
Heat a heavy frying pan and sear your piece of meat on all sides, remove. Heat the oil in the same pan and saute the spring onions, garlic and ginger until softened. Add the soy sauce, half of the coriander, and the chilies. Place the seared meat in a roasting tin. Pour over the fried vegetable mixture. Add the lemongrass and pour the coconut milk and lime juice over the meat. Roast for 40 minutes.
Place beef on a serving dish. Strain the sauce, add the rest of the chopped coriander and pour over the meat. Serve immediately.