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After a 10-year hiatus comma Mrk 590 has rekindled its nuclear activity and is now strongly flaring in a low X-ray state. This offers a very rare opportunity to follow in real time the onset of AGN activity that can lead to better insights on the AGN central engine physics. While directly impacting galaxy evolution through energetic feedback comma the AGN structure and physics are still poorly understood. We wish to catch the early onset of AGN activity to test details of AGN accretion physics that cannot be constrained in any other way. We propose a ToO program to obtain four COS spectra of Mrk 590 at different X-ray flux levels as its AGN builds up. We will measure UV broad-line fluxes comma constrain the UV and EUV ionizing continuum emission from the accretion disk comma and establish the critical ionizing luminosity required for broad-line production comma something that is still V1.0. This allows us to test theoretical model predictions for the broad-line region physics. The efficient strategy we employ ensures that instructive constraints are obtained with a very modest HST program - yet comma the full time request may not be needed. HST is the only telescope that can obtain spectra of the far-UV emission below 1900A that are crucial to reach our science goals. By combining this HST program with observations obtained with Swift comma NuSTAR comma XMM and VLT we will provide a comprehensive account of the black hole accretion state changes during the early onset of AGN activity as the AGN builds up. These HST-COS data will contribute with unique information to the legacy database on this intriguing object that will undoubtedly spawn new insight on AGN physics and the changing-look AGN phenomenon.