to make this work, i needed to add my shell to the /etc/shells file. then i issued the chsh to make my shell change.
$ chsh -l
$ sudo chsh -s /usr/bin/xonsh mock
Changing shell for mock.
my previous post about xonsh is what led me here. i had manually changed the shell directly in the /etc/passwd file, but then my gnome session did not recognize my user. this was the strange side effect of changing the default shell not on the /etc/shells list. gnome (gdm) started the gnome-initial-setup process which runs when gnome is freshly installed.