To cook fish, you must be raiding a dungeon with complexity level 2+. To cook potatoes and fillings, you must be raiding a dungeon with complexity level 5+. Much of the food found in the tunnels beneath Daemonheim is already cooked, but it is possible to catch fish from some of the dark pools within the structure, and, for this reason, it is useful to be able to prepare them using the Cooking skill. As all the ingredients are edible (with the exception of raw fish; mushrooms and potatoes can be eaten raw), remember to right-click when making filled potatoes.