I am using fontina in a spinach pie. Best way to prepare it? Grating versus food processor?

I hate hand grating cheese - I always end up with grated knuckles. For a dish like a spinach pie, I usually use the grating disk on my food processor - quick, does a good job, and there is no triage involved.

If the cheese feels too soft, you can pop it in the freezer for a few minutes to firm it up for grating.

Either will work, grating by hand will satisfy you better.