always here-
I think “overnight (potato) kugel” is any potato kugel that’s left in the oven at low heat for several hours – often overnight on Friday for the Shabbos daytime seuda – after it’s finished cooking.
This causes the kugel to turn a brown or brownish-yellow color instead of the usual yellow potato-kugel color, and the flavor changes in a way that many people (including yours truly) find delicious. (Of course, regular potato kugel is also just fine.)
Potato kugel can either be of the “overnight” variety or not.
So can flanken kugel – it can be cooked like a regular non-overnight kugel, or for an extended period at low heat beyond that for more of an “overnight” taste.