On the premise that Galatians was written after Paul's first missionary journey, when he visited the lower region of the province of Galatia (Acts 14), but before the Jerusalem Council (Acts 15):

To what "prior writing" (proegraphē) was Paul referring in Galatians 3:1 which speaks of Christ as crucified?

You foolish Galatians, who has bewitched you, before whose eyes Jesus Christ was publicly portrayed (proegraphē) as crucified? (Galatians 3:1 NASB95)

English Bibles consistently translate proegraphē as "publicly portrayed" in Galatians 3:1, but this is inconsistent with how the various forms of prographō are translated in the rest of the New Testament, although some may take Jude 4 as referring to a figurative prior writing. Further, the instances of this word which I've assessed in the LXX and in Greco-Roman writings can all be understood as referring to something which is previously written, painted, etc., including those instances where something is "set forth as a public notice," per the LSJ lexicon. Although, admittedly, some instances are ambiguous. [If you are aware of an instance where the word definitely does not refer to something which was previously written, then please let me know! I address a number of Greco-Roman uses of the word in my book: https://atrustworthygospel.com/the-book-is-now-available/]

Note: the stated premise about the timing of the letter to the Galatians is asserted by Bruce, Lonenecker, and Schreiner. I can provide proper footnotes, if desired.