  Electronic correlation is of fundamental importance to high temperature
superconductivity. While the low energy electronic states in cuprates are
dominantly affected by correlation effects across the phase diagram,
observation of correlation-driven changes in fermiology amongst the iron-based
superconductors remains rare. Here we present experimental evidence for a
correlation-driven reconstruction of the Fermi surface tuned independently by
two orthogonal axes of temperature and Se/Te ratio in the iron chalcogenide
family FeTe$_{1-x}$Se$_x$. We demonstrate that this reconstruction is driven by
the de-hybridization of a strongly renormalized $d_{xy}$ orbital with the
remaining itinerant iron 3$d$ orbitals in the emergence of an orbital-selective
Mott phase. Our observations are further supported by our theoretical
calculations to be salient spectroscopic signatures of such a non-thermal
evolution from a strongly correlated metallic phase into an orbital-selective
Mott phase in $d_{xy}$ as Se concentration is reduced.
