The emonPi is not geared up for this out of the box, as Bill states the case physically restricts using the hdmi socket and if you remove the emonPi from it’s case (or cut a hole) the emonSD doesn’t have the desktop packages installed, so you will need to install them and the chromium browser to even view emoncms directly on the Pi as Robert suggests.
Once you have over come the above and you are able to view emoncms in a browser on the hdmi connected monitor then there are further options available to you to autostart the browser in full screen on reboot and even disable the keyboard and mouse (if connected) using a kiosk mode, I prefer not to use kiosk mode so that I can navigate various emoncms pages and even escape kiosk mode into the desktop.
See these links
https://openenergymonitor.org/forum-archive/node/5824.html
https://openenergymonitor.org/forum-archive/node/10444.html
(That last link also explains how to add the desktop packages to the emonSD)