Adjust rack to upper third of oven and place a heavy baking sheet on the rack.
Place all custard ingredients in a food processor or blender.
Process until well combined.
(To mix by hand: In a large bowl, whisk together the egg with the orange zest, orange liqueur, and orange juice. Mix in sour cream, sugar, and sifted flour until the mixture is well blended. Break up any lumps of flour with your fingers.) The custard can be made up to three days in advance and refrigerated.
Arranging the Fruit.
For peaches or nectarine: Do not peel.
Cut the fruit into sixths.
Starting at the outside edge of the tart, lay slices on their sides, touching each other, in two concentric circles on the bottom of the baked tart shell.
Arrange the few remaining pieces, skin side up, in the center to fill in the gaps.
For apricots: Do not peel.
Cut the apricots into fourths.
Arrange the slices in their sides, nestled together, in three concentric circles on the bottom of the baked tart shell.
Place 2 or 3 slices in the center to fill in the gaps.
Filling and baking the tart: Place the custard mixture in a heatproof bowl and stir it over gently simmering water until it is just warm to the touch.
Pour approximately 2 cups of sour cream custard mixture into the tart shell.
The tops of the fruit slices will be poking up through the custard.
Place the tart in the oven on the heated baking sheet and bake for 40-45 minutes.
During baking, the custard will set from the outside edge toward the center of the tart.
Watch carefully during the last 8 minutes or so of baking, jiggling the baking sheet occasionally, so that you can remove the tart from the oven as soon as the custard has set.
(Fruit with a high water content may give off a lot of juice when cooked, which will make the center part of the tart appear very loose. Don’t think that it means that the custard hasn’t set; these juices will be absorbed back into the tart as it) Cool completely.
Slip a cardboard round underneath and remove the flan ring.
Serve at room temperature, with Vanilla Sauce flavored with orange liqueur, or Apricot Sauce.