To marinade the prawns, place all the ingredients for the prawn marinade, minus the prawns, into a small chopper or food processor and blitz until smooth. Mix with the prawns and set aside for twenty minutes.
Heat the oil over medium-high heat in a wok or a high-sided frying pan.
Once hot, fry the onions with cumin, mustard, and curry leaves. Stirring prevents the onions from burning and protects the mustard seeds from scorching, or they’ll turn bitter.
When the onions have begun to turn golden brown, add the garlic, ginger, turmeric, chilli, ground coriander, black pepper, tomato puree, and chopped tomatoes. Add salt to taste.
Cook for 6-7 minutes until the tomatoes break down. Stir frequently to prevent scorching.
Remove the sauce from the stove. Blend until smooth using a hand blender or standing blender. Add a splash of water if it’s too thick to blend.
Return the sauce to the heat, and add the prawns and marinade, cover and cook for 6-7 minutes until the prawns are just cooked.
Add salt to taste and finish with the chopped coriander.