Justifying the correct implementation of the non functional requirements of mission-critical systems is crucial to prevent system failure. The latter could have severe consequences such as the death of people, and financial losses. Assurance cases (e.g., safety cases, security cases) can be used to prevent system failure. They are structured sets of arguments supported by evidence and aiming at demonstrating that a system’s non functional requirements have been correctly implemented. However, although the availability of complete assurance cases is crucial to allow the research community to contribute to the system assurance field, it remains very challenging to access complete assurance cases due to several concerns such as confidentiality issues. Furthermore, assurance cases are usually very large documents. Still, their creation remains a manual, tedious, and error-prone process that heavily relies on domain expertise. Thus, exploring techniques to support their automatic instantiation becomes crucial. To fill these gaps, our experience paper first demonstrates the feasibility of an AMLAS-based design methodology on a case study aiming at manually creating a safety case for the ML-enabled trajectory prediction component of an open-source autonomous driving system i.e. Baidu Apollo. Our paper then reports our experience in using a Large Language Model (LLM) to automatically re-create the same safety case. The lessons we drawn from this case study provide actionable insights that could benefit researchers and practitioners.