Can this dish be assembled ahead of time and then baked a day or two later?

I think you could definitely assemble it several hours ahead of time. If you want to assemble it a couple of days in advance, I'd recommend keeping the sautéed apple-onion-cream mixture separate from the blanched, chopped greens and separate from the cheese and nuts. (This will keep the flavors and layers more distinct!) Then assemble the three component parts when you're ready to bake.