Is week old ratatouille ok?

Ratatouille is like wine. Ages well! But don't be fooled... Onion, a key ingredient, doesn't!

Mine would still be fine as I keep it in the fridge, and it seems to keep there almost forever, or until it all gets eaten up.

As long as it has been refrigerated properly and shows no signs of mold grow and/or an odd odor (any odors other than what it should smell like), then you would be ok. If you are unsure, throw it out, but I think it would be ok since there's no meat, no dairy.