Heat the oil in a medium saucepan over medium heat. Add the mustard seeds and cook for 30 seconds or until the seeds begin to pop.
Add the onion, garlic, curry leaves and stock to the pan. Cover and bring to the boil over medium-high heat. Reduce heat to low, cover and cook, stirring occasionally, for 6 minutes or until the onions are soft.
Meanwhile, deseed and finely chop the chillies. Peel and finely grate the ginger. Cut the fish into 4cm pieces.
Stir the chillies, ginger, coriander, cumin and turmeric into the onion mixture and cook, stirring, for 1 minute or until aromatic.
Stir in the fish and coconut milk, and bring to the boil over medium-high heat. Reduce heat to low and cook, partially covered, stirring gently occasionally, for 5 minutes or until the fish just flakes when tested with a fork.
Combine the yoghurt and cucumber in a small bowl.
Taste and season the curry with salt and pepper. Serve immediately with the yoghurt mixture, rice and pappadums.
