Place the prawns, salt, turmeric and lemon juice in a bowl, mix together and set aside.
In a blender, blitz the chopped onion, ginger, garlic, chillies and chilli powder to a paste.
Heat the oil in a pan over a medium heat, then add the black mustard seeds. As soon as they start to pop, add the onion paste and cook for about 10 minutes, stirring occasionally to make sure it doesn’t burn or stick to the bottom.
Stir in the salt, sugar and turmeric and cook for another five minutes, then add the coconut milk and bring to a simmer.
Add the marinated prawns and cook gently for four to five minutes.
Remove from the heat and allow to rest for 30 minutes, if you have the time. Serve with rice.