I'm trying to understand what the right way is to ask someone what he discussed with someone, or talked about with someone, or talked to someone about. (Like asking what the content or essence of the discussion was).
Marco, it is correct to ask, "What did you discuss with Jeremy?" or "What did you talk about with Jeremy?" You may also ask, "What did you and Jeremy talk about?" or "What did you and Jeremy discuss?"
To my ear, it sounds more fluent to put talk about at the end of the sentence in order to avoid having about and with right next to each other. This is a personal preference and not a grammatical guideline.
Using discuss avoids that issue and so sounds fine to me in either position.
"What did the two of you discuss?" I asked, waggling my eyebrows like I was channeling Groucho Marx.