How to Make It
In a small saucepan, cook 1/3 cup of the sugar over moderate heat, stirring constantly with a wooden spoon, until a dark caramel forms, about 5 minutes. Immediately pour the caramel into the bottom of an 8-by-1-inch ring mold and arrange the prunes on the caramel, spacing them evenly. Let cool completely.
In another small saucepan, combine the milk with 3 tablespoons of the sugar. Scrape the seeds from half of the vanilla bean; add the seeds and scraped bean to the milk and bring to a boil.
Meanwhile, in a medium bowl, vigorously whisk the eggs with 3 tablespoons of the sugar until the mixture turns pale. Gradually whisk in the hot milk. Strain the custard into a bowl and let cool.
Preheat the oven to 325°. Set the caramel-lined mold in a small roasting pan and pour in the custard. Set the pan in the oven. Add enough hot water to the pan to reach halfway up the side of the mold. Bake for about 50 minutes, or until the flan is just set and a knife inserted in the center comes out clean. Let cool in the water bath for 10 minutes, then transfer the ring mold to a rack and let cool to room temperature. Refrigerate the flan for at least 2 hours.
Meanwhile, using a sharp knife, peel the oranges; be sure to remove all of the bitter white pith. Working over a bowl, cut in between the membranes to release the orange segments. Scrape the seeds from the remaining vanilla bean half into the oranges. Toss gently with the orange liqueur and the remaining 2 teaspoons of sugar.
To unmold the flan, bring 1/2 inch of water to a boil in a large skillet. Carefully stand the mold in the skillet for 1 minute, then remove it and wipe the bottom dry. Using a small knife, gently free the flan from the mold. Tilt the mold; if the caramel doesn't seep out, return the mold to hot water and repeat.
Place a serving platter on top of the ring mold and invert the flan onto the platter. Tap the mold lightly and lift it off. Spoon the orange salad into the center of the flan. Slice the flan and serve with the oranges and caramel sauce.