Let's say that your portal is this oval, you could for example use the *Holdout* option of the object so that it creates a hole in it. Render your animation. Now remove the oval and the other objects of your scene, keep the same camera movement and this time shoot the world that is beyond the portal. Then just mix the two footages in the *Compositor* or in the *Video Sequencer*. To have transparency don't forget to enable the *Transparent* option in the *Render panel > Film*.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/XYw1x.jpg