Thermal annealing of thin metal films induces morphology changes that have a dramatic effect in the optical properties. Here we propose an asymmetric Fabry-Perot resonator consisting of a top metal film, a dielectric spacer, and a bottom metal mirror that can display a diverse infrared response. Thermally induced morphology changes result in large reflectivity variations within a limited temperature range following the top film transition between conductive, highly lossy, and transparent regimes. This behavior arises from the combination of the top film properties with interference effects in the multilayer. Furthermore, the proposed structure holds promise for potential sensing applications, as its optical response displays four times larger temperature sensitivity than a single film.