2 large navel oranges
1/3 cup sugar
1/4 cup all-purpose flour
Rounded 1/4 teaspoon salt
3 large eggs, separated
1 cup whole milk
1/2 stick unsalted butter, melted and cooled
2 tablespoons fresh lemon juice
Preheat oven to 350 F with rack in middle. Butter a 1 1/2-quart gratin or other shallow baking dish.
Finely grate 2 teaspoons zest from 1 orange, then squeeze 1/2 cup juice from oranges.
Whisk together sugar, flour, and salt in a large bowl.
Whisk together yolks, milk, butter, zest, orange juice, and lemon juice in a small bowl and add to flour mixture, whisking until just combined.
Beat egg whites in another large bowl with an electric mixer until they hold soft peaks. Stir about one fourth of whites into batter to lighten, then fold in remaining whites gently but thoroughly (batter will be thin).
Pour into baking dish and bake in a hot water bath until puffed and golden, 40 to 45 minutes. Transfer to a rack. Serve warm or at room temperature.