Cook the onions in the oil in a large skillet over medium heat, until soft and golden brown, 12 to 15 minutes.
Add the apples, ½ teaspoon salt, and ¼ teaspoon pepper and cook until just tender, 2 minutes.
Place each sheet of pastry on a parchment-lined baking sheet and prick all over with a fork.
Spread with the crème fraîche, leaving a ½-inch border.
Top with the onion mixture and bake at 400°F until the pastry is crisp and browned, 30 to 35 minutes.
Cut into pieces before serving.
