…small or occasional work outside an artist’s or scholar’s usual specialty? A sideline, a diversion, a one-off foray into an alien subject?
E.g., “Physicist Richard Feynman’s work focused chiefly on quantum theory, but he also produced a [blank] on molecular biology during a sabbatical year.”
Help me fill in the blank! I keep thinking the expression starts with “piece de…” but I can’t get any farther. Is it even a French phrase at all?
Nobody has any ideas? I also thought of “hors d’oeuvre”, but it doesn’t feel right somehow.
Are you thinking of lagniappe? Its most direct meaning is “a little something extra thrown in for free along with a purchase”, but I can see how it could be extended to the sort of thing you describe.
I’m asking my French housemate as I type. 
divertisment?
bagatelle?
essai?
The French housemate wasn’t able to come up with a ‘phrase’ sorry.
Can you give us the sentence in French so far please?
Sorry, there is no French sentence involved: I’m just trying to recall (what I thought was) a French idiom for this particular concept.
Thanks for the help, everybody! Please keep trying; I’m going mildly nuts over this. (Maybe it’s just not a French expression at all?)
I asked the 8 French people in the office and none can think of the expression you’re looking for. Though they are all thinking of the up-coming holidays so maybe it’s just them
Thanks anyway! I am left with the unsatisfactory possibilities “hors d’oeuvre” and “opusculum”.
I shoulda just gone nuts right away and avoided the whole topic.
None of the above. Sideline is un à-côté, or un petit à-côté
(how do you like the accents ?) 
Forgot the source: it’s Harrap’s English-French dict.
(French speaking here)
Gymnopithys
L’accents ?? C’est très magnifique !!!
(Or is it “trés”?)