  The thermoelectric Nernst effect of solids converts heat flow to beneficial
electronic voltages. Here, using a correlated topological semimetal with high
carrier mobility $\mu$ in presence of magnetic fluctuations, we demonstrate an
enhancement of the Nernst effect close to a magnetic phase transition. A
magnetic instability in NdAlSi modifies the carrier relaxation time on
'hotspots' in momentum space, causing a strong band filling dependence of
$\mu$. We quantitatively derive electronic band parameters from a novel
two-band analysis of the Nernst effect $S_{xy}$, in good agreement with quantum
oscillation measurements and band calculations. While the Nernst response of
NdAlSi behaves much like conventional semimetals at high temperatures, an
additional contribution $\Delta S_{xy}$ from electronic correlations appears
just above the magnetic transition. Our work demonstrates the engineering of
the relaxation time, or the momentum-dependent self energy, to generate a large
Nernst response independent of a material's carrier density, i.e. for metals,
semimetals, and semiconductors with large $\mu$.
