The expression 'speaking of...' is most commonly used for introducing a new topic into a conversation that is connected to something just, or at least very recently, mentioned. Switching from one topic to another in the middle of a conversation can seem rude or lead to confusion, but using this expression allows for a much smoother, clearer transition. It is more commonly heard in social situations, but can also be used in an informal professional context, and shares a similarity to the phrases 'by the way,' and 'in/with regard to.'

1. Person A: "I haven't seen Steven for a while. Probably not since his birthday." Person B: "Speaking of birthdays, are you planning to have a party for yours next month?"
2. Person A: "I saw Jenny and her sister at the shops yesterday. I didn't have time to stop and chat, though." Person B: "Speaking of Jenny, did you hear she got fired from her job?"

1. Tom - there's going to be a meeting after lunch to discuss marketing strategies. Speaking of which, have you finished collating the data from the customer surveys we did?
2. While we're speaking of insurance, I have a good broker I can recommend when it is time to renew our policy.