In Romania, they have a saying: “The best vegetable is meat.” Actually, it might be an “old Alsatian saying,” or from some other primeval region, depending on how you Google. But joke like that at a dinner in Brooklyn — or anyplace, really, where identifying with the progressive vanguard has made eating a serious form of self-definition — and someone just might denounce you between forkfuls of heirloom greens. Mealtime, in these rooftop-gardened corners of the world, has become a theater for a kind of culture war. And that evangelist with the fork is hungry for combat, no matter which...