"on a topic" is correct and usable in written English.
It is usually used when talking about a particular subject, idea, or theme. For example, "I wrote an essay on the topic of air pollution."
And they took on a topic that is both edgy and original.
Maybe take one to lunch and start a conversation on a topic of mutual interest.
The system listens and watches for key words on a topic requested by a country.
It's tricky to have a debate on a topic that a participant doesn't think is real.
I am appalled that you would begin a Presidential debate on a topic like that….
Tweeting is no different to writing an opinion piece on a topic.
Then, site searchers were trained to scour each Web site for information on a topic.
A non-fiction tome that helped you become an expert on a topic?
I'm appalled you would begin a presidential debate on a topic like that.
She just wanted to inaugurate a global discussion on a topic politicians often prefer to overlook.
And I am appalled that you would begin a presidential debate on a topic like that".
