"By the way, ..." How to express relatedness of topics in conversation?

Question

In a conversation, one might want to stress that what one is going to say is related to what has been said (right) before.

In English, this can be achieved by starting a sentence with

By the way, ...

or

Speaking of <insert subject matter>, ....

or

Speaking of that, ...

In German, one would usually start a sentence with

Wo wir's gerade davon haben: ....

or

Apropos, ...

(The German word "apropos", like "apropos" in English, derives from the French "à propos". But other than the English one, it is AFAIK never used as an adjective in German, nor with prepositions as in English ("apropos of") or French ("à propos de").)

How is such a stress on the relatedness of the upcoming statement or topic to previous ones expressed in Esperanto conversation?

Answer 1

The Lernu.net dictionary translates parenteze to "by the way". bab.la provides the opposite translation ("by the way" to parenteze). The same translation is used by Duolingo exercises (such as this one).

Parenteze, ŝi havas koramikon.

The reasoning behind parenteze (or "parenthetically") is that the exchange has taken a detour into another (potentially) marginally related subject, yet not necessarily as a continuation of the previous sentence's line of thought, thus the intent of logically wrapping the following sentence around parentheses.

In contrast, cetere translates to moreover, what's more, or besides, which conveys a continuation of the previous line of thought, rather than a focus on relatedness to what has been said.

Whether to use one or the other may well depend on context. parenteze is likely to fit in, although without conveying that it was brought up by a specific matter mentioned earlier, like in "speaking of which".

One can also start a sentence with the direct translation of "about that", pri tio, to focus the attention on something previously said.

Answer 2

Cetere, …

There are various instances in Tekstaro of this usage; just search for \bcetere,. Here are some examples:

“Jes, ekzistas. Iu alia devas rapide salti sur ĝin, eltiri pistolon kaj pafmortigi la ĉevalon, tiam la reĝo estos savita. Sed kiu tion scias! Cetere, kiu dirus tion al la reĝo, fariĝus ŝtono de la fingroj ĝis la genuoj.”

—Fratoj Grimm - Elektitaj Fabeloj

— Kial do vi estas tiel malrapida, tiuvespere? al ŝi diris Matildo.

— Ne pli ol kutime, via Moŝto. Cetere, post unu momento, mi estos fininta. Mi tuj ekfermos la fenestrajn kovrilojn, kaj poste mi eliros.

—Kastelo de Prelongo

Vi eraras — diris la skribisto — li efektive regas. Li havas la povon, la volon, mi nur la scion... Cetere, oni ne batas min, nek vin, kiel tiun kamparanon...

—La Faraono