Preheat oven to 375°.
On a floured work surface, spread open the puff pastry sheet, if folded. Roll to a rectangle a scant ½ inch thick. Line a baking sheet with parchment paper and transfer the rectangle to the paper. Pinch the edges up to form a generous ½-inch rim. Place in the freezer and chill 15 minutes to firm the edges, or refrigerate for 1 hour.
Slice the oranges crossways into ½-inch-thick slices. Do not trim the peels. With the tip of a knife, remove the seeds.
Remove the chilled pastry and lay the oranges on top in a single snug layer. Sprinkle with the sugar.
Bake until the pastry is puffed and browned and the oranges have caramelized slightly, about 30 minutes. Remove from the oven, cover loosely with aluminum foil and let stand for 10 minutes.
Cut the tart into rectangles about 2 inches by 3 inches. Serve warm with the optional crème fraîche.