Yield: 6 servings.
2 tablespoons olive oil
2 cups thinly sliced sweet onions (about 2 medium onions)
1 tablespoon dark brown sugar
2 tablespoons balsamic vinegar
½ teaspoon salt
½ teaspoon freshly ground black or white pepper
1 (13.2-ounce) package puff pastry
4 ounces Gorgonzola or Danish blue cheese, coarsely chopped or crumbled
½ teaspoon fennel seeds
Apple or pear slices (for serving)
Preheat oven to 400°. Heat oil in a Dutch oven or cast-iron skillet over medium-high heat. Add onions and cook for about 10 minutes or until wilted and starting to brown. Add sugar, vinegar, salt, and pepper.
Reduce heat and cook slowly, uncovered, for 20 to 25 minutes or until onions turn caramel-brown in color. If onions start to dry out, add a few tablespoons of water. Remove from heat and let cool.
Roll pastry into a 10- to 14-inch rectangle. Cut pastry into individual 4½-inch round tarts or leave as one whole sheet. Place pastry on a parchment-lined baking sheet. Score the edges of the pastry, half an inch from the outer edge, without cutting all the way through.
Spread cooled onions over pastry. Dot evenly with cheese. Sprinkle with fennel seeds. Bake for 20 minutes or until cheese has melted and pastry is crispy and golden brown. Let cool for 5 minutes. If pastry was baked as one sheet, cut pastry into wedges or squares before serving. Drizzle honey over each serving, if desired, and serve with apple or pear slices on the side.