speak on (something)
To give a lecture or discourse about some topic. The professor will be speaking on the dangers of social media at the conference. Yesterday I spoke on the economic challenges we have been facing. Today, I'd like to go over proactive steps we can take to mitigate those challenges.
speak (up)on something
to talk about a particular topic. (Upon is formal and less commonly used than on.) This evening, I will speak upon the subject of Plato's thoughts on the universe. What will you speak on today?