We have a feature request open on a built-in perspective that produces a list like this, but at the moment, you have to do at least part of it by hand.
(Email the
support ninjas to add yourself to this feature request.)
How to automate the portion that's susceptible to it:
Open a new OmniFocus window on Project view.
Select whichever option under View -> Sidebar Filter meets your needs.
Select View -> Grouping -> Folder
Select Perspectives -> Save Window as New Perspective
Use the window that appears to name the perspective "Flat Project" (or whatever works for you)
Close the perspectives window.
From now on, you can bring this view up by selecting the new item that was added to the perspectives window.
The manual part:
Hold down the option key, and, starting at the top of the outline view on the right of the window, click each folder row in the outline. This collapses the folder and all the projects it contains.
Once you reach the bottom, release the option key. Start at the bottom and click each folder once to open it. Since the option key isn't held down, you'll open the folder but not the projects.