Just as the key ingredient for beef manhattan is left overs, this is a bit of a left over comment. My son heard your show in March and lately asked if I knew what a beef mahattan is. Of course I said EVERYONE knows that.
Well today listening to the episode I discovered that I am wrong and this seems to a be particularly local dish/name. I agree with the probable school cafeteria origin as I can remember these from about 1950 in the Crooked Creek (Indianapolis) school lunches.
I was further surprised at how little info there is on the web. In the entire Google universe there are only 7900 entries for Beef Manhatten. I have heard of "hot brown" but excused that as coming from someone that was just out of touch.
As someone in a blog entry from Ft.Wayne mentioned, "now I am hungry for one".