  Using 3+1 classical lattice simulations, we follow the symmetry breaking
pattern and subsequent non-linear evolution of a spectator field non-minimally
coupled to gravity when the post-inflationary dynamics is given in terms of a
stiff equation-of-state parameter. We find that the gradient energy density
immediately after the transition represents a non-negligible fraction of the
total energy budget, steadily growing to equal the kinetic counterpart. This
behaviour is reflected on the evolution of the associated equation-of-state
parameter, which approaches a universal value $1/3$, independently of the shape
of non-linear interactions. Combined with kination, this observation allows for
the generic onset of radiation domination for arbitrary self-interacting
potentials, significantly extending previous results in the literature. The
produced spectrum at that time is, however, non-thermal, precluding the naive
extraction of thermodynamical quantities like temperature. Potential
identifications of the spectator field with the Standard Model Higgs are also
discussed.
