Preheat the oven to 220°C/200°C Fan/Gas 7. Cut out 23cm/9in circles of baking paper and fold in half.
Lay the tomato slices on one half of the baking paper. Divide the rest of the cod ingredients between the two parcels and season with salt and pepper. Fold over the paper and crimp the edges to seal. Splash the parcel with water then place it onto a large baking tray (or two smaller trays). Bake in the oven for around 8–10 minutes.