Mix all the spices in a little warm water or stock to form a paste.
Heat the coconut oil over a medium heat and cook the onions, garlic and ginger until softened. Add the masala paste and cook for 5 minutes, stirring to prevent burning.
Add the tomatoes and stir well then slowly add the water or stock followed by the potato. Cook over a moderate heat for 20 mins, then add the jaggery, mango, salt and prawns. Cook for 10-15 minutes more.
Serve with rice and peas, accompanied by a cucumber and tomato salad.