Whisk together in a small stainless-steel or enamel saucepan:
Grated zest of 1/2 lemon
Grated zest of 1/4 orange
1/2 cup fresh orange juice
3 tablespoons fresh lemon juice
3 tablespoons water
1/2 cup sugar
2 tablespoons all-purpose flour
1/8 teaspoon salt
3 large egg yolks, or 1 large egg and 1 large egg yolk
Cook, whisking constantly and scraping the bottom and corners of the pan to prevent scorching, over medium heat until the mixture is thickened and begins to bubble. Then continue to cook, whisking briskly, for about 1 minute. Using a spatula, scrape the filling into a medium-mesh sieve set over a bowl and strain the filling into the bowl. Cover the surface of the filling with a piece of wax or parchment paper and refrigerate to thicken. Stir gently if necessary before using; do not beat. This keeps, refrigerated, for up to 2 days.
