"By the way, what are you eating?"
Translation:Cetere, kion vi manĝas?
I contend that "Flanke, kion vi manĝas" or even "aliflanke…, escepte…, krome…, ktp would be more fitting here than "cetere."
flanke – marginally, aliflanke – on the other hand, escepte – exceptionally, krome – besides.
Cetere means "by the way", term that does not refer to the eating but to the situation in which the question is posed.