Preheat the grill to high and arrange prawns on a baking tray.
Toast cumin and coriander seeds in a small pan until they pop, then grind to a coarse powder.
Fry onions in sunflower oil until golden-brown, then stir in spices, garlic, chilli, mango, and fresh coconut for 3 minutes.
Stuff the prawns with a portion of the spice mixture and grill for 3 minutes.
Combine the remaining spice mixture with coconut milk and tamarind paste in a pan and simmer for 2 minutes.
Whisk lime juice and olive oil, then toss with spinach and mint leaves in a bowl.
Plate the grilled prawns with the salad and drizzle with the warm curry sauce.