  The short period ($0.94$-day) transiting exoplanet WASP-19b is an exceptional
target for transmission spectroscopy studies, due to its relatively large
atmospheric scale-height ($\sim 500$ km) and equilibrium temperature ($\sim
2100$ K). Here we report on six precise spectroscopic Magellan/IMACS
observations, five of which target the full optical window from $0.45-0.9\mu$m
and one targeting the $0.4-0.55\mu$m blue-optical range. Five of these datasets
are consistent with a transmission spectrum without any significant spectral
features, while one shows a significant slope as a function of wavelength,
which we interpret as arising from photospheric heterogeneities in the star.
Coupled with HST/WFC3 infrared observations, our optical/near-infrared
measurements point to the presence of high altitude clouds in WASP-19b's
atmosphere in agreement with previous studies. Using a semi-analytical
retrieval approach, considering both planetary and stellar spectral features,
we find a water abundance consistent with solar for WASP-19b and strong
evidence for sub-solar abundances for optical absorbers such as TiO and Na; no
strong optical slope is detected, which suggests that if hazes are present,
they are much weaker than previously suggested. In addition, two spot-crossing
events are observed in our datasets and analyzed, including one of the first
unambiguously detected bright spot-crossing events on an exoplanet host star.
