There are a few Android APPs available for this purpose, such as air display, idisplay, Spashtop.
Their approaches are quite similar: install and run the app (pc version) on your PC, and install and run app (android version) on your Onyx ereader. The ereader and pc must be connected to the network. Then Onyx erader can act as either the mirror of the pc or as the second screen for the pc.
Currently I am trying Spashtop:
http://www.splashtop.com/.
The reason why I am trying this one is that it is free for personal use (Wifi), while others are charging fee.
So far I am able to mirror my pc (window 10) to Onyx ereader by using Spashtop. But the contract rate on Onyx ereader is so poor and I can hardly read it. Maybe it is something wrong with my setting.