Preheat the oven to 425°F and put a large baking pan in the oven to heat.
Take a piece of baking paper about 60–80cm/24–32in long and fold in half to make it double thickness, then fold in half again. Tightly fold together to seal two of the edges, creating a pouch, making sure there are no gaps for the food to escape when cooking. Repeat with another piece of baking paper.
Divide the tomatoes between the parcels, then do the same with the butter beans, olives, red pepper, chorizo and spinach. Season inside the bags with salt and pepper, then put the cod on top of the spinach. Season the fish and finish with a drizzle of extra virgin olive oil. Seal the open end of the bags by folding the edges over, leaving as much space in the bags as possible for steam to circulate when cooking. (The parcels can be prepared in advance and kept in the refrigerator for a good few hours before cooking.)
Put the parcels in the hot baking pan and bake for 20 minutes until the fish is cooked through. When cooked, split open the parcels and serve hot.