After making the filling as directed, I found it to be quite bland. Added way more spices; found the bread to be unnecessary. Took the recommendation of others and served chilled, which I think was the way to go. Won't be making these again.
These were delicious. I especially loved the toasted bread crumb topping and the hit of fresh thyme. I did chill them overnight before serving and I thought they were fantastic. My only complaint is that there was barely enough of the yolk mixture to fill all of the egg whites. I may consider discarding only 2 yolks instead of 4 next time around and then increasing the mayo and mustard as needed.