A search for charged Higgs bosons heavier than the top quark and decaying via is presented. The analysis uses 13.2 of collision data at a centre-of-mass energy TeV collected with the ATLAS detector at the Large Hadron Collider. The production of a charged Higgs boson in association with a top quark and a bottom quark, , is explored in the mass range 300 to 1000 GeV using multi-jet final states with one electron or muon from the top-quark decay. Events are categorised according to the multiplicity of jet and -tagged jets. Multivariate techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and upper limits are set on the production cross section times the branching fraction BR(), together with their interpretation in benchmark scenarios of the Minimal Supersymmetric Standard Model.