The FEAST WP3.3 Co-design workshop handbook provides a comprehensive, practice-oriented guide for facilitators who aim to engage communities—especially vulnerable groups—in co-creating solutions that support healthier and more sustainable food choices. Developed within the Horizon Europe FEAST project, the handbook establishes the rationale for participatory, human-centred design methods in food-system transformation. It explains that effective interventions require not only scientific evidence but also community ownership, shared understanding, and active involvement of those most affected by unhealthy and unsustainable dietary environments. Co-design, drawing on participatory design, cognitive mapping, and adapted elements of Group Model Building, is presented as a powerful approach for eliciting lived experiences, revealing systemic barriers, and fostering collective creativity. The document further emphasises the importance of recognising the multi-level nature of food environments, which span personal, household, community, and policy contexts. The handbook guides facilitators through each step of preparing, conducting, and following up on a three-hour workshop. It outlines how to select participants, ideally a mix of vulnerable community members and relevant stakeholders, and how to create a comfortable, inclusive environment. Detailed facilitation scripts and materials are provided, including an icebreaker (“cabbage game”), templates, and instructions for managing group dynamics. The core of the workshop consists of structured exercises: first, participants create a “graph over time” to reflect on how dietary behaviours have evolved and to collectively formulate an ideal future “dream.” They then identify bottlenecks that hinder healthier and more sustainable eating, mapping these across micro, meso, and macro levels. A complementary exercise asks them to identify and map existing qualities, strengths, resources, practices, or institutions that already support positive dietary behaviours. These steps are designed to shift thinking from isolated problems to positive and constructive systemic relationships. Based on this shared understanding, participants move into solution co-creation. Working individually and in groups, they brainstorm interventions and policy ideas, guided by cards and prompts informed by the Behaviour Change Wheel. They then jointly construct a “pathway” that connects their selected solutions with relevant stakeholders, targeted bottlenecks, and reinforcing qualities. The workshop concludes with a debrief, and instructions to carefully document all outputs and for further development within the local Living Lab. The handbook also provides facilitation tips, troubleshooting advice, templates for informed consent, surveys, and an action-plan tool to support follow-up implementation. Overall, the document offers a clear, adaptable methodology for participatory food-system innovation and community-led action.