For the fish, marinate the fish steaks with the chilli powder, turmeric, lemon juice, green mango and salt, then leave to rest for 30 minutes.
Heat the oil in a large pan set over medium-high heat and shallow-fry the marinated fish for just 2-3 minutes. Remove from the heat and reserve until required.
For the curry, heat the oil in a large pan set over medium heat. Add the cardamom, cassia bark, cumin, fennel and cloves and cook until the whole spices splutter.
Add the onions and cook until softened, then add the tomatoes, ginger, garlic and green chillies. Cook for a further 10 minutes, then add the low-fat coconut milk and the reserved fried fish.
Cook until the sauce becomes a little thicker, then add the full-fat coconut milk and the curry leaves, and simmer for 10 minutes. Remove from the heat, season to taste, and add the fresh coriander.
Serve hot, with plain boiled rice.