The optimal integration of propulsion systems in the next generation of aircraft is a fascinating and challenging multidisciplinary design problem. To fully realise the combined potential of next generation aircraft and engine technologies an integrated multiphysics design approach is key. In this context the German Aerospace Center (DLR), the French Aerospace Lab (ONERA) and Airbus are currently developing CODA (\textbf{C}FD \textbf{O}NERA \textbf{D}LR \textbf{A}irbus), a next-generation CFD solver for aircraft and turbomachinery design, devised to be able to fully exploit current and future HPC architectures~\cite{Goertz2022}. To efficiently simulate turbomachinery components a number of specific boundary conditions are necessary. In this paper we discuss the implementation of the so-called mixing-plane boundary condition in CODA. Due to the relative motion between adjacent blade rows in compressor or turbine stages, the flows within turbomachinery components are inherently unsteady. To nevertheless efficiently simulate such flows in the context of steady simulations, approximate artificial boundary conditions (mixing-planes), in combination with some form of non-reflecting boundary condition, are commonly used between blade rows~\cite{Giles1991}. The implementation of such mixing-plane boundary conditions (which are non-local in space) in an highly optimized HPC environment, such as CODA, is nontrivial. In this work we describe the mixing-plane boundary condition implementation in detail and outline the approach adopted to minimize its impact on code performance and scalability. \begin{thebibliography}{99} \bibitem{Goertz2022} S. G\"ortz, T. Leicht, V. Couaillier, M. M\'eheut, P. Larrieu and S. Champagneux, ``CODA: A European Perspective for a Next-Generation CFD, Analysis and Design Platform," NATO AVT-366 Workshop on Use of Computational Fluid Dynamics and Analysis: Bridging the Gap Between Industry and Developers, 16-19 May 2022. \bibitem{Giles1991} M. Giles, 1991, ``UNSFLO : A Numerical Method for Unsteady Inviscid Flow in Turbomachinery," Tech. Rep., Gas Turbine Laboratory Report GTL~205, MIT Dept. of Aero. and Astro. \end{thebibliography}