Marinate the shrimp with the ingredients listed under 'marinade' and keep it in the refrigerator.
In a wok on the stovetop, add the oil and bring to a heat.
Once hot, add the mustard seeds. Once the mustard seeds start to crack and fizzle.
Add the chopped ginger and garlic, saute until garlic is lightly browned.
Add the onions and saute until onions are translucent. add the curry leaves and sauté for a minute. Add the spice powders, followed by the sliced tomatoes.
Cook with a lid on the wok for 5-6 minutes, stirring occassionally, until the tomatoes are softened.
Add the marinated shrimp and cook, mixing gently with the masala for 5-7 minutes, until shrimp is just cooked. Do not overcook.
To make the tempering, heat coconut oil in a small saucepan. Add the mustard seeds and heat until they sputter. Then add the curry leaves, heat for a minute more. Pour this tempering over the cooked prawn masala, and serve.