Let eggs stand at room temperature for 30 minutes. In a large bowl, combine sugar and flour. Stir in the milk, lemon juice, butter and lemon zest. Beat egg yolks; add to the lemon mixture. In a small bowl and with clean beaters, beat egg whites on high speed until stiff peaks form. Fold into lemon mixture.
Pour into a greased 1-qt. baking dish. Place in a large baking pan. Fill larger pan with hot water to a depth of 1 in. Bake, uncovered, at 350° for 45-50 minutes or until a knife inserted in the center comes out clean. Serve warm. Refrigerate leftovers.