I think the subject is you but it's left out. (You should) recall the idea we were using is the following:
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Gleb_ChebrikoffGreetings, Zepengli,
Your sentence is most probably incomplete, so it is not clear what its boundaries are. If it were
Recall the idea we were using.
or if it were written with a comma or a colon:
Recall: the idea we were using is the following...
recall would be an imperative - it would