  We study the semi-inclusive production of real, high-$p_T$, isolated photons
in unpolarized and polarized lepton-proton collisions, $\ell p\to \ell\gamma
X$. In particular we analyze the transverse nucleon single-spin asymmetry
within the collinear twist-3 formalism in perturbative QCD to leading order
(LO) accuracy. We find that this spin asymmetry is generated by twist-3
dynamical quark-gluon-quark ($qgq$) correlations in the nucleon through the
so-called soft-fermion and hard pole contributions. Hence, this process
unprecedentedly allows for a point-by-point scan of the support of the
dynamical $qgq$ twist-3 matrix elements $F_{FT}(x,x^\prime)$ and
$G_{FT}(x,x^\prime)$ in lepton-nucleon scattering experiments.
