  The Higgs boson can mix with a singlet scalar that dynamically generates the
Majorana mass of the right-handed neutrino $N_R$. We show that even a tiny
mixing between the Higgs boson and a `decoupled' singlet scalar allows for
Higgs-mediated pair production of $N_R$ without significant mixings between the
active neutrinos and $N_R$, and thus testable at colliders via a characteristic
signal of two same-sign same-flavor lepton pairs, plus missing energy. We
demonstrate that this search channel is mostly background-free in
$pp$-collision and can be a highly sensitive probe of the Higgs-singlet mixing
at the current and future $pp$ colliders. Such channel provides a clean signal
to discover the singlet scalar and explore the origin of neutrino masses.
