  This paper addresses the transition from the normal to the superfluid state
in strongly correlated two dimensional fermionic superconductors and Fermi
gases. We arrive at the Berezinskii-Kosterlitz-Thouless (BKT) temperature
$T_{\text{BKT}}$ as a function of \emph{attractive} pairing strength by
associating it with the onset of "quasi-condensation" in the normal phase. Our
approach builds on a criterion for determining the BKT transition temperature
for atomic gases which is based on a well established Quantum Monte Carlo
analysis of the phase space density. This latter quantity, when derived from
BCS-BEC crossover theory for fermions, leads to non-monotonic behavior for
$T_{\text{BKT}}$ as a function of the attractive interaction or inverse
scattering length. In Fermi gases, this implies a robust superconducting dome
followed by a long tail from the flat BEC asymptote, rather similar to what is
observed experimentally. For lattice systems we find that $T_{\text{BKT}}$ has
an absolute maximum of the order of $0.1 E_F$. We discuss how our results
compare with those derived from the Nelson Kosterlitz criterion based on the
mean field superfluid density and the approach to the transition from below.
While there is agreement in the strict mean-field BCS regime at weak coupling,
we find that at moderate pairing strength bosonic excitations cause a
substantial increase in $T_\text{BKT}$ followed by an often dramatic decrease
before the system enters the BEC regime.
