I've read this one :) It's called A Belly Full of Fire by FruitPastilles.
I was able to find it because this was such a unique fanfic that I remembered part of the summary, "he breathed fire". Here's the summary:
When people asked Hisashi what his Quirk was, he'd flash a grin and say he breathed fire. It was a small lie, easier to say than 'my human skin ...
Despite the sparseness of the question and details, I actually believe I read this a few weeks ago. I've read many BNHA fanfictions and hadn't actually seen this strategy before, which is why it stuck out to me when I read it.
The Fanfic Is:
Live a Hero by BeyondTheClouds777
Here are some excerpts from Chapter 11:
A robot spins around the corner, and ...