I’m fairly new to Cubase and I was searching for a way to switch from monitoring in stereo to monitoring in mono and I found it.
It’s located in control room.
You get to control room by either selecting Devices > Control Room Mixer, or through mix console by selecting the top left button and checking the “control room/meter” box.
When control room is visible, look under the “Speakers” box. Here you can select through different monitors and you can change your monitoring from “stereo” to “mono” by clicking on the word “stereo”.
If you are looking for a key command that does this, there isn’t one. You have to assign it.
Go to File > Key Commands… > Control Room > Select Next Downmix Preset and assign a key command of your choice.
I hope this is helpful.