The Most Powerful Lenses in the Universe: Quasar Microlensing as a Probe of the Lensing Galaxy

We describe how quasar microlensing is the only way to determine the dark/stellar ratio at the interesting sub-galactic scales of several kpc and how it is the only way to measure the stellar M/L outside the solar neighborhood. Both can be done as a function of redshift, and this requires sub-arcsecond X-ray imaging of LSST-discovered lenses.