Preheat oven to 400 degrees, with rack in bottom third of oven. Preferably using an electric knife, cut the oranges crosswise into 1/8-inch-thick slices, discarding any seeds. In a medium saucepan, combine orange slices, water, and sugar over medium heat. Cook until orange slices are tender and translucent, about 30 minutes. Remove from heat, and let cool to room temperature.
Line a baking sheet with a cooling rack. Remove the orange slices from the cooking liquid and place on rack to drain. Return the reserved cooking liquid to the stove and cook over medium-high heat until reduced and slightly thickened, 5 to 10 minutes.
Roll out puff pastry to form an 11-by-14-inch rectangle. Lightly sprinkle a baking sheet with water. Transfer pastry to baking sheet. Trim edges to form a 10-by-13-inch rectangle. Using a paring knife, lightly score the pastry one-inch from the edge, being careful not to cut all the way through. Spread frangipane evenly over pastry, within the border.
Top frangipane evenly with drained orange slices. Transfer tart to oven and bake for 45 minutes, rotating baking sheet every 15 minutes for even cooking.
Immediately slide the tart off the baking sheet onto a cooling rack. Lightly brush orange slices with a little of the reduced cooking liquid; discard remaining liquid. Cool tart to room temperature before serving. Serve with creme fraiche, if desired.