Cut the beef into small pieces and wash well. Strain the water. Heat coconut oil in a pan and sauté onion slices. When it becomes brownish add chopped ginger, garlic, green chilly and curry leaves. Stir the ingredients for 2 minutes. When the oil separates, add chilly powder, coriander powder, pepper powder, turmeric powder and stir them for a minute. Add tomato pieces, potato pieces and beef into it. Add little salt to taste and mix the ingredients well. Cover and cook it on low flame for an hour. ( 1/2 hour if it is in a pressure cooker). When the beef gets cooked add the coconut milk  and heat it for some more time. When it starts boiling remove from the stove. In a separate pan, heat the coconut oil and splutter the mustard seeds. Add curry leaves and the small onions pieces. When the mixture becomes brownish add it to the beef curry. Serve the curry along with rice, appam or chapathi.