This custard gets its velvety texture by being baked in a bain-marie–a shallow hot-water bath–for slow and even heating.
Melt ⅓ cup sugar in a heavy saucepan over medium-low heat until it turns golden brown. Carefully pour the caramel into a 1-½-quart casserole dish or divide it among eight individual ramekins (or custard cups) and set aside.
Bring a kettle of water to a boil and preheat oven to 350°. Meanwhile, in a mixing bowl, beat eggs and sugar with an electric mixer until pale yellow. Stir in salt, milk, and lemon zest; mix well. Pour the mixture into casserole dish or divide among ramekins. Next, pour boiling water into a 2-quart (or larger) baking dish. Set casserole dish or ramekins into this larger pan and place in the oven. Bake 50 to 55 minutes at 350°, until centers no longer jiggle when tapped. Let cool. To serve, invert pan on a plate and let the carmelized sugar sauce ooze over the flan.