discourse (up)on (something)
To talk about something at length. If he discourses about this abstract theory any longer, I'm going to fall asleep.
discourse (up)on
someone or something to lecture about someone or something. (Upon is formal and less commonly used than on.) I would like to discourse upon this matter awhile. The committee chose to discourse on Tom and his latest fiasco rather than deal with the budgetary problems it faces.