learned or learnt

[Eve]

Work question...

post a project implementation we conduct a review with all parties to see where things could have gone better etc and any lessons we need to take away for future work.

Is it 'Lessons Learned' or ' Lessons Learnt' ??

I think either is OK and the only difference is that "learnt" is only used in British English.
OED

either is okay, but learned is better for writing and learnt for speaking.
if you're using informal writing (rather than an essay for example) then learnt is fine there too.
i would use Learned in your case :)

Or "chiefly" in British English at least.

"Learned" (one syllable, verb) is what we American folks use; British seem to use "learnt".

The adjective "learned" (two syllables, pronounced ler-ned) is the same in both British and American English, if I'm not mistaken.