Why Did the Duchess Potato Collapse?
Substituting olive oil for butter may have been part of the problem. Olive oil is considerably thinner. &quot;Fresh cream&quot; may also have been problem, since (depending on country) things labeled as &quot;cream&quot; may contain significantly more moisture than heavy cream.
I don't think that letting the potatoes sit overnight would have been a problem. In fact, I would suspect it would work in your favor. But the big, heavy, cold glass dish would really work against you. The way these things are meant to &quot;work&quot; is that the surface rapidly dries and gels, keeping the shape while the heat spreads to the inside. In a metal dish, the bottoms would have heated and dried first, because the large area of the dish would have conducted heat to them. But your refrigerated glass dish would have held down the temperature on the bottom of the potatoes for much of the cooking process, as the mixture heated and thinned.
EDIT: I also see in comments that you left out the egg yolks. Don’t do that. They’re necessary.