Open the coconut can without shaking it. Remove the cream at the top. Save it. Now add enough water to the can to fill it to the top again. Put this thin coconut milk into a bowl.
Put the chillies, turmeric, fresh coriander, pepper, ground coriander and ground cumin into the container of an electric blender along with 4tbsp of the thin coconut milk. Blend. Pour this spice mixture into the bowl with the thin coconut milk.
Put the poppy seeds into a clean coffee grinder and grind as finely as possible. Add to the thin coconut milk.
Heat the oil in a deep frying-pan over a meduim-high heat. When hot, put in the onions, garlic and ginger. Stir and fry until they brown at the edges. Now pour in the seasoned thin coconut milk and add the tamarind, salt and 120ml/4fl oz/½ cup water. Cover and simmer gently for 10 minutes. Add the prawns (shrimp) and the reserve thick coconut cream. Mix well and bring to a simmer. As soon as the prawns (shrimp) turn opaque all the way through, they are done.