I was re-reading Dark Age by Pierce Brown and was reminded of a scene where Mustang reminisces about her time working for the Sovereign. I looked through both Morning Star and Iron Gold but I couldn’t find it for the life of me.
To the best of my memory, it’s Mustang telling Darrow about one of the afternoons she had there. I think a main point of the scene is her sharing her relationship with Cassius (revealed in Golden Son) with Darrow because it’s a thing in the past that probably continues to hurt him. I think at some point she talks about sex and then she mentions that Lysander walks into the room, she describes him as small, blonde, and inquisitive.
If you remember the book that would be great but even better if you could provide the page numbers. Thanks!