1. Wash the large bunch of spinach, leaving the stems on. 
2. Bring a saucepan of salted water to the boil, then hold onto the root end of the stems and plunge the spinach leaves into the water for 30 seconds. 
3. Remove and lay the stems flat on a tea towel to cool. Pick the leaves off the stems, taking care to flatten them.
4. Season the 4 x 200g fillets of fish with a little salt, then take 4 pieces of plastic wrap and spread a few of the leaves out on each one. 
5. Place a piece of fish on top of the spinach, then wrap the fillet tightly in the plastic wrap. Repeat the process. Fill the bottom of a steamer with water and bring it to a simmer. 
6. Place the fish on the steamer tray and place it over the simmering water. Cook until the fish is ready - about 8 minutes - then remove from the heat.
7. Unwrap the fish and set aside to keep warm.
8. Clean the mussels in their shells thoroughly under cold running water, then remove the beards. 
9. Steam the mussels open in a scant amount of water over a high heat for 3 minutes. Remove the shells, then set the mussels aside, reserving the juices in a bowl.
10. Heat a large frying pan or wok, then add the curry paste. 
11. Cook until it becomes fragrant, then stir in a small amount of the coconut milk until the paste dissolves in the milk - it will look as if it has split, but this is okay.
12. Add the fish sauce and sugar, the remaining coconut milk, a splash of the mussel cooking liquid and the chillies (seeded and chopped).
13. Cook until the sauce thickens - about 4 minutes - then return the mussels to the pan along with the herb leaves.

Red Curry Spice Paste

1. Remove the stalks from the chillies, then chop the flesh. Place the chillies and lemongrass in a blender. 
2. Shred the lime leaves and add to the blender with the coriander root and garlic.
3. Grate the ginger, then add to the blender with the shrimp paste.
4. Blitz until smooth, adding a couple of tablespoons of water if required. 
5. Excess paste can be stored in the refrigerator.