Position a rack in the lower third of the oven. Preheat the oven to 375°F. Butter and sugar a 2-quart soufflé dish (see About Dessert Soufflés).
Melt in a saucepan over medium heat:
3 tablespoons unsalted butter
Whisk in until smooth:
1/4 cup all-purpose flour
Cook, stirring, for 1 minute. Remove from the heat and stir in:
1 cup light cream or 1/2 cup each whole milk and heavy cream
1/2 cup sugar
Grated zest of 2 lemons
Bring just to a boil, whisking constantly, and remove from the heat. In a large bowl, whisk until slightly thickened:
5 large egg yolks
Very gradually whisk in the cream mixture, then stir in:
1/3 cup strained fresh lemon juice
Beat on medium speed until foamy:
6 large egg whites, at room temperature
Add and beat until soft peaks form:
1/2 teaspoon cream of tartar
1/8 teaspoon salt
Increase the speed to high and beat until the peaks are stiff but not dry. Using a large rubber spatula, gently stir one-quarter of the egg whites into the egg yolk mixture, then fold in the remaining mixture. Turn the batter into the prepared dish and smooth the top. Bake in a water bath (see About Baked Custards) until a skewer inserted sideways in the center comes out clean or just slightly moist, 1 to 1 1/4 hours. Serve at once with:
Rich Hot Lemon Sauce or Lemon Sabayon
The soufflé can be set aside at room temperature, covered with an inverted bowl, for up to 1 hour before baking.
