Mettle Mantra VR > Mirrors: Simulates one or more flat mirrors of arbitrary size and distribution. It reflects the opposite side of the enclosing sphere.
Charles Yeager (yeagerfilm.com) leads us through the effect in this tutorial.
Mantra VR is a 360/VR Stylization Plugin for After Effects and Premiere Pro.
More info + Free demo at mettle.com/product/mantra-vr/