Chiral polariton lasing is a source of circularly polarized, energy-efficient coherent emission. However, generating high optical contrast of opposite handedness is challenging because excitonic gain materials show low circular dichroism at room temperature. In addition, highly chiral lasing requires the symmetry of the optical cavities to be broken, which can affect the resonance quality and result in low-chiral purity. Here, we report how plasmonic nanoparticle lattice cavities having mismatched dimer unit cells strongly coupled to CdSe nanoplatelets can facilitate polariton lasing with low threshold fluences (8 μJ/cm