Coat the fish in the cumin, coriander and turmeric. Heat the oil in a heavy-based saucepan over a low heat and fry the fish, until golden on all sides. Remove from the pan and set aside.
Add the onion, garlic, chilli, ginger, tomatoes, cumin seeds and mustard seeds to the pan and fry for 6–8 minutes. Add the stock and simmer for 10 minutes. Put the fish back in the pan and poach for 6–8 minutes. Add the coriander and season with salt and pepper.
To make the naan bread, mix the yeast, sugar and milk in a bowl. Stir in the yoghurt, flour, baking powder and salt and knead for 10 minutes. Cover the bowl and leave to prove for an hour, until risen. Knock back and kneed for another 5 minutes.
Put a bakng tray under the grill and preheat to medium.
Take a small ball of dough, about 50g/1¾oz, and roll into a circle shape on a floured surface. Put the grated paneer into the centre and pinch the dough together, then roll into an oval shape. Brush with melted butter and sprinkle over the mustard seeds. Repeat with the remaining dough.
Grill the naans on the hot baking trays for 10–15 minutes, until puffed up and golden.
To serve, ladle the curry into four bowls and serve with the naan breads.