Heat the oven to 350°F. Grease a 1½-quart baking dish.

Whisk the egg yolks, sugar, flour, and salt in a bowl.

Stir in the milk, lemon juice, zest, and butter.

Beat the egg whites until soft peaks form. Fold them gently into the lemon mixture.

Pour into dish. Set in a larger pan; add hot water halfway up the sides.

Bake forty-five minutes until lightly golden. Serve warm or chilled.