From lemons, grate 1 tablespoon peel and squeeze 1/2 cup juice.
In large bowl, combine sugar and flour. With wire wisk, beat in milk, egg yolks,melted butter, and lemon peel peel and juice.
In small bowl,with mixer at high speed, beat egg whites and salt until soft peaks form when beaters are lifted. With rubber spatula, fold one fourth of the egg whites into lemon mixture; gently fold in remaining egg whites just until blended. Pour batter into prepared baking dish.
Place baking dish in a small roasting pan; place on rack in oven. Carefully pour enough boiling water into roasting pan to come halfway up sides of dish. Bake until top is golden and set, about 40 minutes (batter will separate into cake and pudding layers). Cool in pan on wire rack 10 minutes. Serve warm.