I cook beef heart quite often, but haven’t tried sous vide yet. I found this in my searches, which looks promising:
I normally do beef heart in a raw (unglazed) clay pot (Römertopf). That works well, but the heart can still come out a bit chewy. I’m thinking of doing the sous vide thing on the heart first, for maybe twelve hours or so (with minimal seasoning), and then pretend that it’s still raw and use my usual recipe in the clay pot. The idea is that it should come out more tender that way.