Otter's Rock

I don't think you're being fair, here: For background: I'm a math geek with one Jewish and one non-Jewish parent, raised none-of-the-above, got curious about what it meant to be Jewish as a young adult, now absolutely identify as such, although not very observant. (I always knew my mom was raised Jewish, but it wasn't particularly relevant to my childhood.)

I agree that Steve would be unlikely to assume that Tony is Jewish just because of Howard's upbringing. His mom, after all, was names Maria, which is on the short list of least Jewish girl's names. But it's not at all implausible for Steve to assume that Tony knows that Howard was, originally, Jewish: it could be a discussion of different religions with Thor, for example, and for Tony to be totally surprised. Given that new knowledge, I could easily see Tony being curious about what "being Jewish" means. Based on my experience, and those of others like me I have known, he'd be more likely to go down the rabbit hole of Jewish rabbinic discussion and laws, than the heritage (Polish, Russian, etc.) I can easily see him, say, having a conversation with JARVIS while he's puttering that ends up chasing the kosher food laws back to "Don't seethe a goat in its mother's milk," and trying to figure out how we got here from there. There's a lot there that's fascinating to a geek-type mind, no matter what you think of it.

I'm not saying this is a great idea to write: I know that my opinions about a lot of Jewish laws, which I could easily see putting into Tony's mouth, would upset other, more observant, Jews. But my TL;DR: having Tony explore the religion, as an obsessive outsider suddenly curious about what "being Jewish" means, seems entirely plausible. And hey, writing a seder with all the Avengers could be a lot of fun.