This is why no gentile should ever write that fic, because I would not know where the line is. Obviously, from this exchange.

I wasn't actually necessarily thinking that the fic would be about Tony being offensive about adopting Jewish culture. I was thinking more about Steve assuming that Tony was Jewish, or at least knew that he had Jewish heritage, and that reaction there. Tony being a dick about it would follow if you extended it. Tony is a dick about everything, much as fandom tends to make him a woobie. I mean, yeah, there is genuine emotional depth there, and Tony might genuinely want to search for his heritage, but he would cover it with asshole, as he pretty much always does.