Although the pie is traditionally Cornish I'm not sure how authentic this is. However it is the one I remember from my childhood and which I rediscovered in my mother's very old and tatty recipe book. I have tweaked it a bit here and there and I'm flexible about the vegetable filling.
Other versions can be made using mackerel or pilchards, but sardines are traditionally used. I used green beans in this pie because that was what I had to hand, but you could use peas or carrots or anything else seasonal. Whatever you use, cut fairly small and lightly steam until tender.