Notes: The onion-apple topping can be made up to 2 days ahead. The triangles can be prepared, covered, and chilled the morning of a party, then popped into the oven when guests arrive. They can also be frozen, wrapped well, for up to 3 months (do not defrost before baking).
3 tablespoons unsalted butter
1 pound yellow onions, thinly sliced
1/2 tsp salt
2 Granny Smith apples, peeled, cored, and thinly sliced
1/4 teaspoon freshly ground black pepper
1 sheet frozen puff pastry, defrosted just before use
3 ounces fontina cheese, shredded
1 tablespoon minced fresh thyme leaves
Melt butter in a large frying pan over medium-high heat. Add onions and 1/2 tsp. salt. Cook, stirring occasionally, until onions are soft, about 5 minutes. Add apples and stir to coat. Reduce heat to medium and cook, stirring often, until onions are medium brown, about 30 minutes. Stir in pepper and salt to taste. Set aside and let cool, about 10 minutes.
Preheat oven to 375°. Unwrap puff pastry sheet. Cut sheet into 16 squares, then halve those squares diagonally to make 32 triangles. Arrange them, not touching, on a baking sheet.
Place a spoonful of cooled onion-apple mixture on each triangle. Top with a sprinkle of shredded cheese and minced thyme. Bake until puffed and golden, 15 to 20 minutes. Serve hot.
Note: Nutritional analysis is per triangle. Yield: Makes 32 hors d'oeuvres. CALORIES 72 (60% from fat); FAT 4.8g (sat 1.6g); CHOLESTEROL 6mg; CARBOHYDRATE 5.9g; SODIUM 77mg; PROTEIN 1.4g; FIBER 0.5g