  In this work we generalise the adjoint method of inverse design to
nonreciprocal media. As a test case, we use three-dimensional topology
optimization via the level-set method to optimise one-way energy transfer for
point-like source and observation points. To achieve this we introduce a suite
of tools, chiefly what we term the `Faraday-adjoint' method which allows for
efficient shape optimization in the presence of magneto-optical media. We carry
out an optimization based on a very general equation that we derive for energy
transfer in a nonreciprocal medium, and link finite-different time-domain
numerics to analytics via a modified Born series generalised to a tensor
permittivity. This work represents a stepping stone towards practical
nanophotonic optical isolation, often regarded as the `holy grail' of
integrated photonics.
