One of the ways to look for evidence of dark matter (DM) at a collider experiment is through s-channel processes where a DM mediator decays to two quarks. At ATLAS, analyses looking for di-jet resonances are limited to mediator masses above a TeV, due to the high transverse momentum (pT) requirements of jet triggers. However, sub-TeV mass regions can be explored if the resonance is produced with a large relativistic boost provided by an initial state radiation jet. B-tagging the final state can further reduce backgrounds for mediators with democratic decays to all quarks; this final state also allows access to Higgs boson production through gluon-gluon fusion, which at high Higgs pT can be increased to up to 50% by the presence of BSM couplings. This talk will give an overview of the strategy and first results of this kind of study at ATLAS, both for a scalar DM mediator and Higgs interpretations.