Mesh objects are assumed to represent movable objects and are not used for the sprawling level geometry typically used to create backgrounds.
So how do I make world geometry? This feel like an oversight where the authors assumed that the answer was obvious, but I really can't find it. Googling questions like "how do I make a cube?" or "how do I make a room with walls and a floor?" leads me to answers that boil down to "Use your SceneManager to generate a surface mesh, then build walls and stuff with it" which strike me as extremely tedious and inefficient.
tl;dr What is the simplest, recommended way to make static non-file-based geometry in Ogre3d?