This paper presents Project PHoENIX, which stands for Participatory, Human-centered, Equitable, Neurodiverse, Inclusive, eXtended reality. The project aims to co-produce research with autistic users to create a virtual reality (VR) environment that is highly usable, accessible, and sensitive to the needs and preferences of these individuals. Project PHoENIX utilizes participatory design within a learning experience design (LXD) frame to locate autistic people, their caregivers, and providers centrally in the processes of immersive technology design and development, as well as research design and execution. An overarching literature review on VR and autism and issues of limited design precedent of VR environments with autistic participants is provided, as well as details on the Project PHoENIX design framework, project description, and project design outcomes. Details are provided on how the online VR environment was co-designed and co-developed through collaborative research with autistic stakeholders while being sensitive to their needs and preferences. Research findings and implications are discussed regarding the design process, constraints, principles, and insights. The paper concludes by discussing lessons learned and how this project can provide much-needed design precedent for advancing the field towards a more inclusive, human-centered, and neurodiverse VR research and development paradigms.