  Large-gap quantum spin Hall insulators are promising materials for
room-temperature applications based on Dirac fermions. Key to engineer the
topologically non-trivial band ordering and sizable band gaps is strong
spin-orbit interaction. Following Kane and Mele's original suggestion, one
approach is to synthesize monolayers of heavy atoms with honeycomb coordination
accommodated on templates with hexagonal symmetry. Yet, in the majority of
cases, this recipe leads to triangular lattices, typically hosting metals or
trivial insulators. Here, we conceive and realize "indenene", a triangular
monolayer of indium on SiC exhibiting non-trivial valley physics driven by
local spin-orbit coupling, which prevails over inversion-symmetry breaking
terms. By means of tunneling microscopy of the 2D bulk we identify the quantum
spin Hall phase of this triangular lattice and unveil how a hidden honeycomb
connectivity emerges from interference patterns in Bloch $p_x \pm ip_y$-derived
wave functions.
