I'm not fluent by any means, so if any native speaker wants to correct me, please do! I would also like to learn.
But anyways, the way I would say this is...
고구마에 대해서 말하고 있었어.
Here, the grammatical form that means "about" is "~에 대해(서)", which you attach to nouns.
The "~고 있다" form, in case you do not know about it, is simply the progressive from (I am doING, I am eatING, etc.), conjugated into the past tense.
I hope I was of help! ^-^