We present a first evaluation of a Serious Slow Game Jam (SSGJ) methodology as a mechanism for co-designing serious games in the application domain of cybersecurity, to evaluate how the SSGJ contributed to improving the understanding of cybersecurity. To this end, we engaged 13 participants with no experience in cybersecurity, in a multidisciplinary SSGJ involving domain-specific, pedagogical, and game design knowledge, and encouraged engagement in-between scheduled days of the SSGJ. Findings show improved confidence of participants in their knowledge of cybersecurity (from 12.5% to 62.5%) after undertaking the SSGJ, with free-text answers specifically indicating an improved understanding in terms of vulnerabilities, attacks, and defences for three quarters of the participants. Also, confidence in knowledge of game design improved (12.5% to 75%), and the SSGJ successfully engaged participants in-between scheduled days. Finally, a serious game is presented that was co-designed with participants during our SSGJ, and produced as an output of the SSGJ methodology.