  We discuss composite UV completions of the Nelson-Barr(NB) solution to the
strong CP problem. In our construction, the CP symmetry is broken spontaneously
by the dynamics of a hidden QCD at $\theta=\pi$. We focus on the minimal
implementation of the NB construction where the visible sector contains one
extra pair of vector-like up/down quarks. We show that the minimal NB theory
suffers from a quality problem, and discuss how composite UV completions may
resolve it. We present a simple calculable scheme, free of a quality problem,
where dynamical CP violation in the hidden sector is mediated through a scalar
portal to the visible sector, which successfully realizes the NB construction.
