what should be the parting line for this cross section (plastic part)?
You could have a hole in the "bottom" face of the part. Doing so will allow the undercut to be formed from the other side, thus no longer making it an undercut.
I'm sure it won't work for your application, but it is the best option to keep tooling costs and complexity low.
What's up with the cross hatching in the image? Is the part curving in both directions? The lines on the "right" side look really odd.