  We present a multi-wavelength study of the apparently non-repeating, heavily
scattered fast radio burst, FRB 20221219A, detected by the Deep Synoptic Array
110 (DSA-110). The burst exhibits a moderate dispersion measure (DM) of
$706.7^{+0.6}_{-0.6}$ $\mathrm{pc}~\mathrm{cm}^{-3}$ and an unusually high
scattering timescale of $\tau_{\mathrm{obs}} = 19.2_{-2.7}^{+2.7}$ ms at 1.4
GHz. We associate the FRB with a Milky Way-like host galaxy at
$z_{\mathrm{host}} = 0.554$ of stellar mass $\mathrm{log}_{10}(M_{\star,
\mathrm{host}}) = 10.20^{+0.04}_{-0.03} ~M_\odot$. We identify two intervening
galaxy halos at redshifts $z_{\mathrm{igh1}} = 0.492$ and $z_{\mathrm{igh2}} =
0.438$, with low impact parameters, $b_{\mathrm{igh1}} = 43.0_{-11.3}^{+11.3}$
kpc and $b_{\mathrm{igh2}} = 36.1_{-11.3}^{+11.3}$ kpc, and intermediate
stellar masses, $\mathrm{log}_{10}(M_{\star, \mathrm{igh1}}) =
10.01^{+0.02}_{-0.02} ~M_\odot$ and $\mathrm{log}_{10}(M_{\star,
\mathrm{igh2}}) = 10.60^{+0.02}_{-0.02} ~M_\odot$. The presence of two such
galaxies suggests that the sightline is significantly overcrowded compared to
the median sightline to this redshift, as inferred from the halo mass function.
We perform a detailed analysis of the sightline toward FRB 20221219A,
constructing both DM and scattering budgets. Our results suggest that, unlike
most well-localized sources, the host galaxy does not dominate the observed
scattering. Instead, we posit that an intersection with a single partially
ionized cloudlet in the circumgalactic medium of an intervening galaxy could
account for the substantial scattering in FRB 20221219A and remain in agreement
with typical electron densities inferred for extra-planar dense cloud-like
structures in the Galactic and extragalactic halos (e.g., high-velocity
clouds).
