Surely no need to cook a fish already cured in vinegar?
If it was me I would buy another unopened jar, preferably not from the under a £1 stall and eat them a la mode.
I wouldn't risk it with fish. Three days being ill in an Acapulco hotel after eating a dodgy bit of Octopus and two weeks in hospital in Oz with amoebic dysentry after eating a dodgy oyster are life lessons learned.