  A bright optical flare was detected in the high-redshift ($z=2.133$) quasar
CGRaBS J0809+5341 on 2014 April 13. The absolute magnitude of the object
reached $-30.0$ during the flare, making it the brightest one (in flaring
stage) among all known quasars so far. The 15 GHz flux density of CGRaBS
J0809+5341 monitored in the period from 2008 to 2016 also reached its peak at
the same time. To reveal any structural change possibly associated with the
flare in the innermost radio structure of the quasar, we conducted a pilot very
long baseline interferometry (VLBI) observation of CGRaBS J0809+5341 using the
European VLBI Network (EVN) at 5 GHz on 2014 November 18, about seven months
after the prominent optical flare. Three epochs of follow-up KaVA (Korean VLBI
Network and VLBI Exploration of Radio Astrometry Array) observations were
carried out at 22 and 43 GHz frequencies from 2015 February 25 to June 4, with
the intention of exploring a possibly emerging new radio jet component
associated with the optical flare. However, these high-resolution VLBI
observations revealed only the milliarcsecond-scale compact "core" that was
known in the quasar from earlier VLBI images, and showed no sign of any
extended jet structure. Neither the size, nor the flux density of the "core"
changed considerably after the flare according to our VLBI monitoring. The
results suggest that any putative radio ejecta associated with the major
optical and radio flare could not yet be separated from the "core" component,
or the newly-born jet was short-lived.
