In a medium bowl, combine ginger garlic paste, turmeric, and chilli powder. Toss in shrimp to coat well. Allow to marinate while preparing remaining ingredients.
In a large pan, drizzle oil over medium high heat. Add mustard seeds and cook just until fragrant, about 30 seconds. Stir in onion and garlic. Cook, stirring often, until softened. Mix in turmeric, chilli powder, and coriander.
Stir in tomatoes and cook until well softened and beginning to release juices. If still too dry, add up to 1/4 cup water. Stir in the coconut.
Add the marinated prawns and cover the pan. Cook, stirring occasionally, just until the prawns are pink and heated through.
Remove from heat, sprinkle with cilantro, and serve immediately with bread or rice.