Drain the liquid from the cans of tuna, and put the tuna into a bowl. If you find a good quality tuna in olive oil, preserve and use a bit later, you can halve the mayonnaise.
Break up the tuna with a fork.
Mix together mayo, mustard and a bit of the preserved olive oil.
Follow with finely chopped celery, spring onion, capers and dill or fennel and mix thoroughly.
Add the feta at the end in order to gently mix it in without smearing it into goo