If a window, door or room opens onto something it has a view of it or opens in its direction.
Examples of use:
1. Their windows open onto the surrounding countryside and they have fantastic views.
2. The hotel entrance opens onto an enormous marble-tiled reception.
infinitive – open onto
present simple – open onto and opens onto
-ing form – opening onto
past simple – opened onto
past participle – opened onto
Image by charles.bucowsky