For the prawns, preheat the grill to high and place the prawns on a baking tray.
Heat a small frying pan and add the cumin and coriander seeds and cook until they start to pop. Remove from the pan, transfer to a pestle and mortar and pound the seeds to a coarse powder.
In a large pan, heat the sunflower oil. When hot add the onions and fry for 4-5 minutes, or until deeply golden-brown. Add all the spices, garlic, chilli, mango, fresh coconut and cook for 2- 3 minutes.
Spoon some of the spice mixture into the centre of the prawns and grill for 3 minutes.
To make the sauce, in a clean pan add the rest of the spice mixture and the coconut milk and bring to the boil, then add the tamarind paste and simmer for 1-2 minutes.
To make the spinach salad, place the spinach and mint leaves in a large bowl. Put the lime juice and oil in a small bowl and whisk, then pour over the salad.
Serve the prawns on a serving plate with the salad alongside and pour over the sauce.