Fast radio bursts (FRBs) carry ‘fossil’ information about non-Gaussianity generated during inflation. This primordial signal is most accessible on the largest scales, where the scale-dependent bias correction $$\propto f_\textrm{NL} H_0^2/k^2$$

\propto f_{NL} H_{0}^{2} / k^{2}

dominates, but where cosmic and astrophysical systematic effects are also strongest. A central obstacle to extracting $$f_\textrm{NL}$$

f_{NL}

from FRB dispersion measures (DMs) is the degeneracy between the intergalactic-medium (IGM) electron bias $$b_e$$

b_{e}

and the primordial non-Gaussianity (PNG) signal, which degrades $$\sigma (f_\textrm{NL})$$

σ (f_{NL})

by factors of 1.7–3.3 when $$b_e$$

b_{e}

is marginalised, severely limiting the constraining power of any single-bin DM analysis. We introduce a new method to break this degeneracy internally, by exploiting the cross-power spectrum $$C^{D\Delta t}_\ell $$

C_{ℓ}^{D Δ t}

between the FRB DM field and the Shapiro timing delays measured along multiple interferometric sightlines. The DM field traces the biased electron density $$\delta _e = b_e\,\delta _m$$

δ_{e} = b_{e} δ_{m}

, while the Shapiro timing signal probes the Newtonian gravitational potential $$\Phi \propto -\delta _m/k^2$$

Φ \propto - δ_{m} / k^{2}

independently of astrophysical bias. Their cross-correlation, when combined with the timing auto-spectrum as a ratio estimator $$\hat{b}_e(\ell )\equiv C^{D\Delta t}_\ell /C^{\Delta t\Delta t}_\ell $$

{\hat{b}}_{e} (ℓ) \equiv C_{ℓ}^{D Δ t} / C_{ℓ}^{Δ t Δ t}

, isolates $$b_e$$

b_{e}

independently of the amplitude of $$P_{mm}$$

P_{mm}

, providing a self-calibration of the electron bias from the FRBs. We derive $$C^{D\Delta t}_\ell $$

C_{ℓ}^{D Δ t}

analytically in the Limber approximation and demonstrate that corrections to the Limber condition for the timing field are suppressed below $$10^{-6}$$

10^{- 6}

at all multipoles of interest, eliminating a potential source of systematic error. A joint Fisher matrix analysis over $$\varvec{\theta } = \{f_\textrm{NL},\,b^0_e,\,z_\textrm{fb}\}$$

θ = {f_{NL}, b_{e}^{0}, z_{fb}}

, using the complete data vector $$\{C^{DD}_\ell , C^{\Delta t\Delta t}_\ell , C^{D\Delta t}_\ell \}$$

{C_{ℓ}^{DD}, C_{ℓ}^{Δ t Δ t}, C_{ℓ}^{D Δ t}}

, shows that including $$C^{D\Delta t}_\ell $$

C_{ℓ}^{D Δ t}

reduces $$\sigma (b^0_e)$$

σ (b_{e}^{0})

by a factor of 2.1–5.1 relative to a DM-only analysis, depending on survey depth and interferometric baseline. After full marginalisation over the bias model, the joint analysis recovers $$\sigma (f_\textrm{NL})$$

σ (f_{NL})

within a factor of 1.0–1.9 of the fixed-bias benchmark, compared with a factor of 1.7–3.3 degradation when the cross-spectrum is omitted. While the resulting single-bin forecasts ( $$\sigma (f_\textrm{NL}) \approx 790$$

σ (f_{NL}) \approx 790

–1360 for $$N_\textrm{FRB} = 10^4$$

N_{FRB} = 10^{4}

, $$l = 500\,\textrm{AU}$$

l = 500 AU

) remain weaker than current large-scale structure constraints ( $$\sigma (f_\textrm{NL}) \approx 20$$

σ (f_{NL}) \approx 20

–50 from galaxy clustering), this paper is primarily a methodological contribution: we establish $$C^{D\Delta t}_\ell $$

C_{ℓ}^{D Δ t}

as a new, internally self-calibrated observable for FRB surveys, and provide the Fisher machinery needed to incorporate it into future tomographic analyses.

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