Bit surprised this morning when I went to try this and LG's website says Macs aren't supported with their screen sharing feature. Couldn't see anything that would easily achieve this either.
TV is a recent model:
https://www.lg.com/uk/tvs/lg-55UM7400PLB
TV is a recent model:
https://www.lg.com/uk/tvs/lg-55UM7400PLB