In the first person storytelling mode we're experiencing, it makes sense that Harry is reluctant to divulge every detail of his last confrontation with his ward and mentor but when can we expect to see it? Is this the big choice that Harry makes in the "Mirror, Mirror" book? To follow Justin down the left hand path? Or later to succumb to the wolf belt in "Fool Moon?"
Jim's plot tease for MM was that the divergence started with the "big choice at the end of Grave Peril".
The readers have, to some extent, disagreed on exactly which big choice that was. The leading candidates were whether to tell Susan he loved her, whether to accept Ortega and Bianca's deal to give up Susan in exchange for peace, whether to fight or flea at Bianca's masquerade (which is stretching the definition of 'end' of the book since it's only about the 2/3 mark) or possibly whether to trust Thomas when he returns Amoracchius.
That last seems like less of an obvious big choice, but I've argued the case that it's potentially at least as consequential in terms of setting Harry's path afterward as the others.
As for a full flashback of Harry roasting Justin, though, no you haven't missed one. JB hasn't detailed that, and there's probably some reason. I'm hoping it's not a "Justin faked his death" reveal, though.