I was going to say a browned butter sauce until I saw your health conscious note!

I, personally, am turned off by the butternut squash/Parmigiano combination, but that doesn't mean everyone else would be!
What about a mushroom based sauce or even a red bell pepper based sauce? The more I think about it the more I like the red bell pepper sauce idea. That would compliment the spinach & the Parmigiano very nicely.
(Or yellow bell pepper for that matter!)