This paper presents a search for physics beyond the Standard Model targeting a heavy resonance visible in the invariant mass of the lepton-jet system. The analysis focuses on final states with a high-energy lepton and jet, and is optimised for the resonant production of leptoquarks — a novel production mode mediated by the lepton content of the proton originating from quantum fluctuations. Four distinct and orthogonal final states are considered: e+light jet, μ+light jet, e+b-jet, and μ+b-jet, constituting the first search at the Large Hadron Collider for resonantly produced leptoquarks with couplings to electrons and muons. Events with an additional same-flavour lepton, as expected from higher-order diagrams in the signal process, are also included in each channel. The search uses proton-proton collision data from the full Run 2, corresponding to an integrated luminosity of 140 fb$^{−1}$ at a centre-of-mass energy of $ \sqrt{s}=13 $ TeV, and from a part of Run 3 (2022–2023), corresponding to 55 fb$^{−1}$ at $ \sqrt{s}=13.6 $ TeV. No significant excess over Standard Model predictions is observed. The results are interpreted as exclusion limits on scalar leptoquark $ \left({\overset{\sim }{S}}_1\right) $ production, substantially improving upon previous ATLAS constraints from leptoquark pair production for large coupling values. The excluded $ {\overset{\sim }{S}}_1 $ mass ranges depend on the coupling strength, reaching up to 3.4 TeV for quark-lepton couplings y$_{de}$ = 1.0, and up to 4.3 TeV, 3.1 TeV, and 2.8 TeV for y$_{sμ}$, y$_{be}$, and y$_{bμ}$ couplings set to 3.5, respectively.[graphic not available: see fulltext]