Tomato and tamarind fish curry

• 500gr cod fillets
• 2 tsp coriander seeds
• 1 tsp cumin seeds
• 1 tsp chilli flakes
• 1 tsp ground turmeric
• 1½ tbsp tamarind paste
• 3 garlic cloves, crushed
• 2cm piece fresh ginger, peeled and chopped
• 2 tbsp mild-flavoured oil (to make a paste and to fry)
• 2 onions, sliced
• 2 tsp black mustard seeds
• 8 dried curry leaves
• 3 medium potatoes (about 400gr) cut into wedges
• 400gr tin chopped tomatoes
• 300ml fish stock
• 200ml coconut cream

To serve:

• Small bunch fresh coriander, leaves picked
• 1-2 tbsp desiccated coconut, toasted in a dry pan
• Cooked jasmine rice
• Lime wedges