Mesoscopic photonic crystal based mirrors are exploited to theoretically conceive and analyse a novel high-Q factor Fabry-Perot-like cavity. Thanks to a beam focussing effect in reflection these mirrors efficiently confine and stabilise a mode inside a planar cavity, even for non-paraxial small beam sizes, mimicking the behaviour of curved mirrors. Furthermore, these mirrors show a higher reflectivity with respect to their standard distributed Bragg reflector counterparts, which allow these cavities to reach a maximum Q factor higher than 104 when 5-period-long mirrors are considered. Moreover, these cavities show other intriguing features as a beamforming behaviour and transverse translational invariance offered by the planar geometry. The latter opens interesting possibilities for lasing and biodetection. The optimization of the cavity size and the performances in terms of Q factor, energy storage and confinement are detailed.