We analyze the prospects for resonant di-Higgs production searches at the LHC in the $bbar{b} W^+ W^-$ ($W^{+} to ell^{+} u_{ell}$, $W^{-} to ell^{-} bar{ u}_{ell}$) channel, as a probe of the nature of the electroweak phase transition in Higgs portal extensions of the Standard Model. In order to maximize the sensitivity in this final state, we develop a new algorithm for the reconstruction of the $b bar{b} W^+ W^-$ invariant mass in the presence of neutrinos from the $W$ decays, building from a technique developed for the reconstruction of resonances decaying to $tau^{+}tau^{-}$ pairs. We show that resonant di-Higgs production in the $bbar{b} W^+ W^-$ channel could be a competitive probe of the electroweak phase transition already with the datasets to be collected by the CMS and ATLAS experiments in Run-2 of the LHC. The increase in sensitivity with larger amounts of data accumulated during the High Luminosity LHC phase can be sufficient to enable a potential discovery of the resonant di-Higgs production in this channel.
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