Despite the growth of urban food policies across the globe, a key challenge remains around measuring the impact of these initiatives in building more sustainable and just cities. The literature identifies as the main barriers to progress food system assessments the lack of clear definitions of sustainability, insufficient data, the low applicability of global conceptualisations to local conditions and vice versa and low levels of actor involvement. We aim to address these gaps by co-developing a sustainability assessment framework to evaluate food systems performance in UK cities. The framework emerging from this collaborative process overcomes key limitations of previous exercises by providing a place-based and participative definition of sustainability aligned with global conceptualisations, building on the experiences and needs of a wide range of practitioners and taking a holistic but non-prescriptive approach to understanding food system outcomes. However, its application to the city of Cardiff reveals new challenges, mostly regarding the need to incorporate underrepresented stakeholders as well as account for multiscalar food system interdependencies and their positive but also negative impacts. Results show the need to embed critical perspectives in sustainable food assessments that actively expand their transformative capacity by developing further inclusive, participatory, place-based and whole-systems approaches.