Household food waste plays a crucial role in the overall global food waste problem. A significant portion of this waste consists of edible leftovers that could be repurposed instead of discarded. However, leftover reuse remains an underexplored strategy for reducing food waste, despite its potential to contribute to more sustainable food consumption. By framing leftover reuse as a sufficiency-driven behavior, this study positions it as a key strategy for reducing food waste and fostering responsible consumption. Following a mixed method approach consisting of qualitative interviews, a survey and creative sessions with designers, this research explores the psychological factors that influence individuals' intentions to reuse food leftovers and provides practical design strategies to encourage leftover reuse. Based on a dataset of 244 households in the Netherlands, we use Structural Equation Modeling (SEM) to test a conceptual model grounded in psychological constructs, assessing factors influencing intention to reuse leftover food. The findings reveal that attitudes, and personal norms are significant factors in encouraging reuse of leftover food, while perceived health risks negatively affect attitudes towards reuse. Based on qualitative and quantitative insights, six evidence-based design strategies were developed to address both psychological motivators and practical challenges. The results contribute to the literature and practice by highlighting the psychological determinants of food leftover reuse and proposing interventions to foster sustainable food consumption practices.