The phrase "back on the subject" is correct and usable in written English.
You can use it when you want to return to addressing a particular topic or idea that was previously discussed. For example, "Now that we've discussed the new project, let's get back on the subject of the upcoming meeting."
12.21pm: Cameron is back on the subject of sport now.
(And no, I'm not back on the subject of Lindsay Lohan).
But soon Mr. Rudolph was back on the subject of his performers.
Since we're back on the subject of Mr. Roth: At 56, he continues to convey both a busy work ethic and an insistent effortlessness.
And back on the subject of health care, Paul Krugman explains the latest Republican attempt to take away health insurance from a lot of people.
Back on the subject of "kleos", there's delicious irony in the fact that men so wrapped up in their own reputations can be so silly.
As the first federal effort since 1997 to examine instruction and measure student achievement in music and the arts, the survey has added new evidence to the debate about whether American schools are cutting back on the subjects they teach to concentrate on improving students' basic skills.
Professor Jackie Eales, president of the Historical Association, said her greatest concern was that the new curriculum, which she likens to the tongue-in-cheek reworking of English history in the 1930s book, 1066 and All That, will lead to children turning their backs on the subject in their droves.
The time to pull the curtain back on this subject is long overdue.
It was all I could do to get them back on to the subject of the film.
There were as many ambient prejudices on the part of critics and sophisticated viewers back then on the subject of what constitutes good acting as there are now.
