  Hund's rule coupling ($\textit{J}$) has attracted much attention recently for
its role in the description of the novel quantum phases of multi orbital
materials. Depending on the orbital occupancy, $\textit{J}$ can lead to various
intriguing phases. However, experimental confirmation of the orbital occupancy
dependency has been difficult as controlling the orbital degrees of freedom
normally accompanies chemical inhomogeneities. Here, we demonstrate a method to
investigate the role of orbital occupancy in $\textit{J}$ related phenomena
without inducing inhomogeneities. By growing SrRuO$_3$ monolayers on various
substrates with symmetry-preserving interlayers, we gradually tune the crystal
field splitting and thus the orbital degeneracy of the Ru $\textit{t$_2$$_g$}$
orbitals. It effectively varies the orbital occupancies of two-dimensional (2D)
ruthenates. Via in-situ angle-resolved photoemission spectroscopy, we observe a
progressive metal-insulator transition (MIT). It is found that the MIT occurs
with orbital differentiation: concurrent opening of a band insulating gap in
the $\textit{d$_x$$_y$}$ band and a Mott gap in the
$\textit{d$_x$$_z$$_/$$_y$$_z$}$ bands. Our study provides an effective
experimental method for investigation of orbital-selective phenomena in
multi-orbital materials.
