Place the tomato purée in a bowl.
Add salt, sugar, garam masala, cumin, chilli powder, coriander, green chilli, lemon juice, and 15 ml of water to the bowl and mix.
Gradually stir in the coconut milk or cream and set the mixture aside.
Heat the vegetable oil in a wok or frying pan over high heat.
Add the mustard seeds and wait for them to begin popping.
Add the garlic and curry leaves, stirring until the garlic turns medium brown.
Add the prawns and stir until they turn opaque.
Pour in the sauce and simmer over medium heat until the prawns are cooked through.
Remove the pan from the heat.