Speaking of which is said to move on to a related subject. Speaking of which, did the plumber ever call.
It uses the last thing said as a bridge, which makes a change of subject sound natural. Without a real link it is an obvious device and everybody notices.
1phrasespeaking of whichSaid to move on to a related subject.
“Speaking of which, we still owe them for the tickets.”
speaking of which is a phrase.
Practised in Small Talk & Everyday Phrases