It sounds like what you're describing is just a continent that's higher in the north than in the south. All you need there is a continental collision where your 'plateau' continent starts riding up over the one it's colliding with. That's exactly how the Himalayas happened, and unless I'm missing something, the entire Indian Subcontinent is a perfect example of what you're asking about.
The other mechanism that traditionally forms plateaus is volcanic, and we have real-world examples of that right here on Earth too. The Thulean Plateau existed about sixty million years ago and was formed by massive eruptions by the volcanic hotspot that's currently under Iceland. I'm not sure you could realistically get a plateau as large as what you're describing that way, because the kind of tectonic activity required to barf up that much magma would also tend to break up the landscape rather than leaving you with the single contiguous landform you're talking about.