There are various ways. Here are a couple.
Suja'chuqtaHvIS ghaH SoH je, nuq bop ja'chuqghach? While you and he were conferring, what was the conversation about?
Soj bopbogh ja'chuqghachraj yIngu' Identify the matter your conversation was about! (You can use Soj food idiomatically to mean subject, matter.)