Prep time: 25 minutes
Cooking time: 45-50 minutes
- 1 tablespoon unsalted butter
- 2 tablespoons olive oil
- 2 medium onions, very thinly sliced
- 1 teaspoon salt (preferably kosher or sea salt)
- 1/4 teaspoon pepper
- 1 teaspoon chopped fresh thyme, plus 1 tablespoon of leaves for garnish
- 2 tablespoons balsamic vinegar
- 1 sheet puff pastry (14 oz, 11 x 13 inches), preferably Dufour Brand, defrosted in the refrigerator (keep refrigerated until ready to use)
- 2 ounces blue cheese, crumbled (scant 1/2 cup)
- Preheat the oven to 400˚F. Line a large baking sheet with parchment paper or spray it with cooking spray.
- Melt the butter with the olive oil in a large skillet over medium heat. Add the sliced onions, salt and pepper; stir well to coat the onions in oil. Cook, stirring often, until dark golden in color, about 18-22 minutes. Add the chopped thyme and cook 30 seconds. Add the balsamic vinegar and scrape up any browned bits on the bottom of the pan. Stir until the vinegar is completely distributed and absorbed. Transfer the caramelized onions to a bowl. Do Ahead: The onions can be covered and refrigerated for up to 3 days. Bring to room temperature before proceeding with the recipe.
- On a lightly floured surface, roll out the puff pastry to 1/8-inch thick. Transfer to the prepared baking sheet. Brush the edges of the pastry with water and fold them over to create a 1-inch rim. Using a fork, poke the dough all over (not on the rim).
- Bake the pastry for 15 minutes or until it is light golden and puffed. Remove the pan from the oven and arrange the onions evenly over the top (inside the rim), deflating the puff in the middle. Scatter the cheese over the onions. Bake 20-22 minutes longer, or until deep golden brown around the edges and under the onions. Turn the oven off, but leave the tart inside for 10 more minutes, allowing the pastry to dry out slightly.
- Let the tart cool 5 minutes. Sprinkle with fresh thyme leaves and cut into squares or wedges using a sharp knife. Serve warm or at room temperature.