Heat the oil in a large, nonstick wok or frying pan and add the shallots. Stir-fry over a medium heat for 10 minutes until lightly golden.
Add the curry leaves, ginger, garlic and chillies. Stir-fry for 1 minute and then add the ground coriander, cumin, mustard, nigella and fennel seeds. Season well and stir-fry for 1–2 minutes.
Stir in the tomatoes and bring the sauce to the boil. Reduce the heat and cook gently for 20–25 minutes, stirring occasionally.
Add the prawns to the pan and cook over a high heat for 3–4 minutes or until the prawns turn pink and are cooked through. Remove from the heat, stir in the chopped fresh coriander and serve immediately with rice.