These are fabulous! I love carmelized onions.
2 Tbs Olive oil
2 medium onions, sliced
2 red apples (gala or braeburn), cut into small pieces
Kosher salt and black pepper
2 sheets frozen puff pastry, thawed
1/2 cup creme fraiche or sour cream
Heat oven to 400. Heat the oil in a large skillet over medium heat. Add the onions and cook, stirring occasionally, until soft and golden brown. Stir in the apples, 1/2 tsp salt, and 1/4 tsp pepper and cook until just tender, about 2 minutes.
Place each sheet of pastry on a parchment lined baking sheet and prick all over with a fork. Spread with the sour cream, leaving a 1/2 inch border. Top with the onion mixture and bake until the pastry is crisp and browned, 30-35 minutes. Cut into pieces before serving.
Make ahead strategy: The tarts can be assembled and frozen up to 1 month in advance. Freeze them, unbaked, on parchment lined baking sheets until firm, then wrap in plastic. To cook, bake from frozen at 400 for 40-50 minutes; keep warm at 250. cut just before serving. From Real Simple magazine.