Peel and finely slice the shallots, garlic and ginger. Trim and finely slice the chilli. Cut the fish into chunks. Heat a lug of groundnut oil in a large pan, add the mustard seeds and curry leaves and cook until the seeds start to pop. Add the shallot, garlic, ginger and chilli, and cook on a medium heat for 5 minutes, or until softened. Mix the chilli powder and turmeric together with a splash of water, and stir into the pan. Fry for 1 minute, then add the fish, coconut milk and tomatoes. Season, bring to the boil, then simmer for 20 minutes, or until the fish is cooked and the sauce has reduced. Pick, chop and scatter over the coriander leaves. Delicious served with basmati rice and warm naan bread.