The increasing demand for sustainable food systems has driven interest in the valorization of plant-based foods and their by-products. Filamentous fungi offer a promising biotechnological tool for transforming low-value biomass into nutritionally enriched or functionally enhanced products while reducing food waste and contributing to the development of a circular food economy. This chapter presents a comprehensive protocol for the fungal valorization of plant-based substrates, detailing the materials, fermentation systems, and cultivation methods. Two primary substrate categories are explored: plant-based foods and their derived industrial by-products. The methodology covers the preparation of substrates, the development of fungal inoculum, and the use of various solid-state fermentation systems (using Petri dishes, containers, or fermentation bags). Specific steps such as biomass quantification, drying, and crushing of the samples are also outlined to ensure reproducibility. Emphasis is placed on the selection of fungal strains and the optimization of growth conditions to maximize valorization efficiency. The protocols and applications described support the development of circular food production models by converting plant-based foods and waste into valuable products, with potential implications as a functional ingredient in the food industry.