In small saucepan over medium heat, dry -fry the cardamom seeds, coriander seeds and mustard seeds,until they begin to jump.Remove from heat, tip into a mortar and grind to a powder with the pestle.set aside.Add the coconut to the saucepan and toast until golden brown.remove from heat and set aside.In a medium frying pan,heat the oil and cook the onion for 5 minutes.Add garlic,ginger,chilies,cumin,turmeric,paprika, garam masala and cook for 2 minutes.Tip in the ground spices,

Toasted coconut and tamarind paste.Transfer to the slow cooker,and add the coconut milk.Stir together to combine.Place the fish and potato in the sauce, cover and cook on low for 4 hours.Thirty minutes before the end of cooking time,add the prawns and stir gently to coat in sauce.Before serving,adjust the seasoning as needed,stir in the coriander and garnish with chopped tomatoes.