  We present the first detailed observational picture of a possible ongoing
massive cluster hierarchical assembly in the Galactic disk as revealed by the
analysis of the stellar full phase-space (3D positions and kinematics and
spectro-photometric properties) of an extended area ($6^{\circ}$ diameter)
surrounding the well-known $\it h$ and $\chi$ Persei double stellar cluster in
the Perseus Arm. Gaia-EDR3 shows that the area is populated by seven co-moving
clusters, three of which were previously unknown, and by an extended and quite
massive ($M\sim10^5 M_{\odot}$) halo. All stars and clusters define a complex
structure with evidence of possible mutual interactions in the form of
intra-cluster over-densities and/or bridges. They share the same chemical
abundances (half-solar metallicity) and age ($t\sim20$ Myr) within a small
confidence interval and the stellar density distribution of the surrounding
diffuse stellar halo resembles that of a cluster-like stellar system. The
combination of these evidences suggests that stars distributed within a few
degrees from $\it h$ and $\chi$ Persei are part of a common, sub-structured
stellar complex that we named LISCA I. Comparison with results obtained through
direct $N$-body simulations suggest that LISCA I may be at an intermediate
stage of an ongoing cluster assembly that can eventually evolve in a relatively
massive (a few $10^5 M_{\odot}$) stellar system. We argue that such cluster
formation mechanism may be quite efficient in the Milky Way and disk-like
galaxies and, as a consequence, it has a relevant impact on our understanding
of cluster formation efficiency as a function of the environment and redshift.
