  Mono-$X$ signatures are a powerful collider probe of the nature of dark
matter. We show that mono-Higgs and mono-$Z$ may be key signatures of
pseudo-scalar portal interactions between dark matter and the SM. We
demonstrate this using a simple renormalizable version of the portal, with a
Two-Higgs-Doublet-Model as electroweak symmetry breaking sector. Mono-$Z$ and
mono-Higgs signatures in this scenario are of resonant type, which constitutes
a novel type of dark matter signature at LHC.
