  Supermassive black holes (SMBHs; $M\gtrsim10^5\msun$) are known to exist at
the centre of most galaxies with sufficient stellar mass. In the local
Universe, it is possible to infer their properties from the surrounding stars
or gas. However, at high redshifts we require active, continuous accretion to
infer the presence of the SMBHs, often coming in the form of long-term
accretion in active galactic nuclei. SMBHs can also capture and tidally disrupt
stars orbiting nearby, resulting in bright flares from otherwise quiescent
black holes. Here, we report on a $\sim200$-s X-ray quasi-periodicity around a
previously dormant SMBH located in the centre of a galaxy at redshift
$z=0.3534$. This result may open the possibility of probing general relativity
beyond our local Universe.
