For objects with features that are symmetrical in the thickness direction, a middle surface can be created using functions from the Generative Shape Design (GSD) workbench. There are several options on how this can be done.

In the following document you can find three scenarios that a user can basically come across:

1. All of the object’s relevant features are symmetrical in the thickness direction.
2. Some of the object’s relevant features are asymmetrical in the thickness direction.
3. It is possible to create the solid from the surface rather than the other way around.

To know it better, please check the methodology in the link.

 Creating an Object's Middle Surface