  Long $\rm \gamma$-ray bursts (GRBs) are produced by the dissipation of
ultra-relativistic jets launched by newly-born black holes after the collapse
of massive stars. Right after the luminous and highly variable $\gamma$-ray
emission, the multi-wavelength afterglow is released by the external
dissipation of the jet in circumburst medium. We report the discovery of a very
bright ($\rm \sim 10$ mag) optical emission $\rm \sim 28$ s after the explosion
of the extremely luminous and energetic GRB 210619B located at redshift 1.937.
Early multi-filter observations allowed us to witness the end of the shock wave
propagation into the GRB ejecta. We observed the spectral transition from a
bright reverse to the forward shock emission, demonstrating that the early and
late GRB multi-wavelength emission is originated from a very narrow jet
propagating into an unusually rarefied interstellar medium. We also find
evidence of an additional component of radiation, coming from the jet wings
which is able explain the uncorrelated optical/X-ray emission.
