On top of the butter goes a portion of whatever was in that bowl besides lemon zest and what was probably some amount of flour. People with a larger food processor (or more spare time than us to do this manually) could probably do the whole job at once.
By "Whole Job" we of course mean cutting together the flour (and whatnot) and butter until the butter turns into pea sized lumps, because that's how good crusts are made.
However, we're not people with more spare time OR a larger food processor than we have (that's just a logical inconsistency), so we have to do it the semi-old-fashioned way: in batches.
