In Luke's account, what did Moses and Elijah discuss with Jesus at the Transfiguration?

Answer: His departure at Jerusalem

They spoke about his departure (KJV: decease) which he was about to accomplish at Jerusalem.

“Suddenly two men, Moses and Elijah, began talking with Jesus. They appeared in glory and spoke about His departure, which He was about to accomplish at Jerusalem.”— Luke 9:30-31 (BSB)