Fry the sliced potatoes.
Make two parcels as follows, using half of the ingredients in each one of them.
Lay out a big sheet of aluminium foil, or baking parchment, and put the potatoes on one of its halves.
Above the potatoes put the tomatoes cut in slices, and above them one of the hake fillets. Season with salt.
Spread the vegetables, finely chopped, over the hake and pour some olive oil.
Fold the paper in half and double fold all around to seal the ingredients.
Bake (200ºC) until the parcels are completely inflated, and the fish will be cooked.
Open the parcel, put on a plate and serve.