The coming decade will mark a milestone in our understanding of interstellar magnetism: achieving a first reconstruction of the Galactic magnetic field in 3D. This will be crucial for progress in fields such as CMB cosmology and cosmic ray physics. Achieving this goal relies on the combination of (a) high-accuracy data that probe interstellar magnetism and (b) novel algorithms that enable the combination of different datasets. A first step towards this direction can be made through the use of starlight polarization in combination with stellar distances. I will discuss PASIPHAE, an upcoming optical polarization survey which aims at producing a tomographic map of the magnetic field orientation in the diffuse ISM. I will present results from a path-finding mini-survey and explore the wealth of information we gain by combining tomographic information with HI surveys in our Galaxy.