Recent years saw advancement in simulations of plasticity, localizations and damage of various alloys and steels, using crystal plasticity or more advanced models. However, their experimental validation, which is often pursued through so-called ‘integrated experimental-numerical testing‘ is not always trivial. Indeed, on bulk metal alloys with fine, complex microstructures, such experimental-numerical comparisons are known to be highly challenging, since measurements are restricted to the sample surface, thereby failing to capture the effects of the 3D subsurface microstructure. As a consequence, a quantitative comparison of deformation fields between experiments and simulations is hardly possible. In this paper, we aim to tackle this challenge by proposing a novel ‘quasi-2D’ integrated experimental-numerical testing methodology. The method hinges on the fabrication of specimens which, in a relatively large region of >100 µm, have a thickness of only a few µm. As a result, the microstructure in this region is practically uniform through the thickness. The specimens are fully characterized from both surfaces and are then tested in-situ to retrieve microstructure-resolved deformation fields. At the same time, the full microstructure is discretized in 3D and simulated. This allows for a detailed, one-to-one quantitative comparison of deformation fields between experiments and simulations, with neglegible uncertainty in the subsurface microstructure. As a result, a degree of agreement between experiments and simulations may be attained which we believe to be unprecedented at this scale. We demonstrate the capabilities of the framework on polycrystalline interstitial-free ferritic steel and dual-phase ferritic-martensitic steel specimens. For both case studies, we will show that the numerical simulations can be quantitatively compared to experiments, thereby allowing to critically assess which advanced modelling choices are required to closely match the experiments.