  We present the long-term spectro-temporal evolution of the average radio
emission properties of the magnetar XTE J1810-197 (PSR J1809-1943) following
its most recent outburst in late 2018. We report the results from two and a
half years of monitoring campaign with the upgraded Giant Metrewave Radio
Telescope carried out over the frequency range of 300 - 1450 MHz. Our
observations show intriguing time variability in the average profile width,
flux density, spectral index and the broadband spectral shape. While the
average profile width appears to gradually decrease at later epochs, the flux
density shows multiple episodes of radio re-brightening over the course of our
monitoring. Our systematic monitoring observations reveal that the radio
spectrum has steepened over time, resulting in evolution from a magnetar-like
to a more pulsar-like spectrum. A more detailed analysis reveals that the radio
spectrum has a turnover, and this turnover shifts towards lower frequencies
with time. We present the details of our analysis leading to these results, and
discuss our findings in the context of magnetar radio emission mechanisms as
well as potential manifestations of the intervening medium. We also briefly
discuss whether an evolving spectral turnover could be an ubiquitous property
of radio magnetars.
