Heat oil in a wok or a large heavy-based frying pan on medium heat. Cook onion for 8-10 minutes or until golden. Add garlic, mustard seeds, curry powder, turmeric, salt and ginger and continue cooking for 2 minutes, stirring constantly, until spices are fragrant.
Stir in coconut milk, shredded coconut, tomatoes, tamarind purée, lemon juice and sugar. Simmer for about 8-10 minutes to allow sauce to reduce slightly.
Add prawns and continue to cook for about 5 minutes or until prawns are just cooked through. Season to taste with more salt if needed but it should be about perfect.
Garnish with some toasted shredded coconut and chilli. Serve with rice.