GPD Win 2 - Need to Rotate Steam, any clue?

I've installed Bazzite (Deck ISO) quite successfully on the GPD Win 2. It boots up fine but is in portrait mode on the built-in screen (which isn't great on a handheld!).

I managed to rotate it in Desktop Mode by plugging in an external monitor, but when I drop back into Steam it's still rotated portrait and there's no option (that I can see) to flip it.

Any ideas anyone please?
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