  A search for heavy resonances decaying into a Higgs boson ($H$) and a new
particle ($X$) is reported, utilizing 36.1 fb$^{-1}$ of proton-proton collision
data at $\sqrt{s} =$ 13 TeV collected during 2015 and 2016 with the ATLAS
detector at the CERN Large Hadron Collider. The particle $X$ is assumed to
decay to a pair of light quarks, and the fully hadronic final state $XH
\rightarrow q\bar q'b\bar b$ is analysed. The search considers the regime of
high $XH$ resonance masses, where the $X$ and $H$ bosons are both highly
Lorentz-boosted and are each reconstructed using a single jet with large radius
parameter. A two-dimensional phase space of $XH$ mass versus $X$ mass is
scanned for evidence of a signal, over a range of $XH$ resonance mass values
between 1 TeV and 4 TeV, and for $X$ particles with masses from 50 GeV to 1000
GeV. All search results are consistent with the expectations for the background
due to Standard Model processes, and 95% CL upper limits are set, as a function
of $XH$ and $X$ masses, on the production cross-section of the $XH\rightarrow
q\bar q'b\bar b$ resonance.
