A spherical mirror is nothing but:

A spherical mirror is a mirror whose reflecting surface forms a part of a hollow sphere. In other words, it is a small piece cut out from a hollow spherical shell, with one side polished (silvered) so that the other side acts as a reflecting surface.

Because the reflecting surface is curved (a portion of a sphere), the mirror has a centre of curvature, a radius of curvature, a pole and a principal axis — features a flat (plane) mirror does not possess. Depending on which side is silvered, the spherical mirror is either concave (reflection from the inner, hollowed surface) or convex (reflection from the outer, bulging surface).

Hence, the correct description is that a spherical mirror is a piece cut from a hollow spherical shell framework.