Not (of course) answering for Santa but my seasoned oil is simply oil reclaimed from curries (I like to make really oily curries, to bring out the flavour, but if I go too far in that direction I just decant a little off and save it), together with the drippings from when I baste my seekh kebabs and the drippings that they spontaneously exude while being cooked (I have a water-filled tray underneath to catch the drippings). Not in a lifetime could I cook sufficient onion bhaji to get my seasoned oil that way. And mustard oil (if you quietly overlook the health warnings and the erucic acid content) adds a certain je ne sais quoi, especially to tandoori and tikka dishes.
** Phil.