First, you’ll want to place some spherical reflection captures around your level.
Placing one close to the mirror will start showing some of the surroundings in the mirror.
Second, you’ll want to turn on clipping plane support in project rendering settings, and place a planar reflection capture that lines up with the mirror. This will make the mirror show more or less what you’d expect it to show.
Third, you’ll want to turn on ray tracing, which will make the scene have the highest quality for reflections.