Preheat oven to 350 degrees F.
Finely grate 1 tablespoon zest from lemons, then squeeze 1/4 cup plus 2 tablespoons juice.
Whisk together flour, salt, and 1/2 cup plus 2 tablespoons sugar in a large bowl.
Whisk together yolks, milk, zest, and juice in a small bowl and add to flour mixture, whisking until just combined. Beat whites in another large bowl with an electric mixer until they hold soft peaks. Beat in remaining 1/4 cup sugar, a little at a time, and continue to beat until whites hold stiff, glossy peaks.
Whisk about one fourth of whites into batter to lighten, then fold in remaining whites gently but thoroughly (batter will be thin).
Pour into a buttered 1 1/2-quart ceramic gratin or other shallow baking dish and bake in a hot water bath until puffed and golden, 45 to 50 minutes.
Transfer to a rack. Serve warm or at room temperature.