  The celebrated Hong--Ou--Mandel effect illustrates the richness of two-photon
interferometry. In this work, we show that this richness extends to the realm
of time-frequency interferometry. Taking advantage of the mathematical analogy
which can be drawn between the frequency and quadrature degrees of freedom of
light when there is a single photon in each auxiliary mode, we consider the
equivalent of the Hong--Ou--Mandel effect in the frequency domain. In this
setting, the $n$-Fock state becomes equivalent to a single-photon state with a
spectral wave function given by the $n^{th}$ Hermite--Gauss function and
destructive interference corresponds to vanishing probability of detecting
single photons with an order one Hermite--Gauss spectral profile. This
intriguing analogy leads us to introduce an interferometric strategy using a
frequency engineered two-photon state allowing to reach Heisenberg scaling for
phase estimation, and to generalise the Gaussian Boson Sampling model to
time-frequency degrees of freedom of single photons.
