The physical region of Middle Tennessee that is an elevated flat expanse of land is the Highland Rim.

Here's why:

* Elevated: The Highland Rim sits at a higher elevation than the surrounding regions, like the Nashville Basin and the Cumberland Plateau.

* Flat expanse: The Highland Rim is characterized by its relatively flat topography, with rolling hills and gentle slopes.

* Land: This region is primarily composed of land, not water, with extensive farmlands and forests.

Let me know if you'd like to know more about Middle Tennessee's physical regions!