How do you set up a 3D object so that when you slice it with the camera's clipping planes, it is solid?

I am attempting to create a toggle-able mode on my camera so that it shows a 2D slice of the 3D objects in the world. I found a question similar to this. In fact, I think it is my exact question, except I do not understand the answer.

My scene is fairly simple: The camera, and a sphere at (0,0,0). If I set the camera’s clipping plane to only show one unit along the z axis at 0. I would like a set up so that a circle is drawn. How would I achieve this?

I’m not sure the camera clip plane is the best method to achieve the look you’re going for.

The fact is, that sphere isn’t solid. Cutting it with the camera’s near clip will just expose the backfaces of that hollow geometry. You could draw it again with a second render pass (like the link you shared) or give them a shader which is a constant value so it appears solid but it won’t have any shape to it.

If you want it to look like it’s actually holding light / has any illumination falloff, you’ll need geometry to ‘hold’ that light, so maybe look at animating the model instead.