Ingredients
Puff Pastry Base
- 1 sheet frozen puff pastry, thawed
- 1 large egg, beaten (for egg wash)
Onion Mixture
- 2 tablespoons olive oil, divided
- 1 medium red onion, thinly sliced
- 2 cloves garlic, minced
- 1 tablespoon balsamic vinegar
Toppings
- 1 tablespoon Dijon mustard
- 2 to 3 medium Roma tomatoes or 2 large heirloom tomatoes, sliced ¼-inch thick
- 8 ounces fresh mozzarella, drained well, patted very dry, and sliced
- ¼ cup Parmesan cheese, freshly grated
Finishing Touches
- ¼ cup fresh basil leaves, torn or thinly sliced
- Flaky sea salt, to taste
- Ground black pepper, to taste
- Balsamic glaze, for drizzling
Instructions
- Preheat Oven and Prepare Baking Sheet: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper to prevent sticking and make cleanup easier.
- Cook the Onions: Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the thinly sliced red onion. Cook, stirring occasionally, until the onions soften and become lightly caramelized, about 12 to 15 minutes.
- Add Garlic and Balsamic Vinegar: Stir in the minced garlic and cook for 1 minute until fragrant. Then add the balsamic vinegar and cook for another 1 to 2 minutes, allowing the liquid to evaporate. Remove the pan from heat and let the mixture cool slightly.
- Prepare Tomatoes: Arrange the tomato slices on paper towels and lightly pat the tops dry to remove excess moisture. Sprinkle lightly with salt, let sit for 10 minutes, then pat dry again to prevent sogginess.
- Prepare Puff Pastry: Place the thawed puff pastry sheet on the prepared baking sheet. Roll it lightly into a 10x14-inch rectangle if necessary. Using a sharp knife, score a 1-inch border around the edges, being careful not to cut all the way through. Prick the center of the pastry all over with a fork to prevent excessive puffing in the center.
- Assemble the Tart: Spread the Dijon mustard evenly over the central area of the pastry within the scored border. Sprinkle the freshly grated Parmesan cheese over the mustard layer. Evenly spread the cooled onion and garlic mixture on top. Arrange the tomato and mozzarella slices in overlapping rows or a decorative pattern over the onions.
- Apply Egg Wash: Brush the exposed pastry border with the beaten egg to encourage a golden, glossy finish when baked.
- Bake: Bake the tart in the preheated oven for 25 to 30 minutes, or until the pastry is puffed and a deep golden brown, and the cheese has melted. If excess liquid appears on top after baking, blot carefully with a paper towel to keep the tart crisp.
- Garnish and Serve: Allow the tart to cool for 5 minutes. Sprinkle with fresh torn basil leaves, flaky sea salt, and freshly ground black pepper. Drizzle with balsamic glaze just before serving for a finishing touch of sweetness and acidity.
Notes
- Patting the tomatoes dry is key to preventing a soggy tart crust due to excess moisture.
- You can substitute fresh mozzarella with burrata or fontina for varied texture and flavor.
- The tart is best enjoyed warm but can be served at room temperature.
- Use a sharp knife to score the border carefully without cutting through the pastry layers.
- For a richer flavor, allow the onion mixture to caramelize until deeply golden brown, but be careful not to burn.
- Prep Time: 20 minutes
- Cook Time: 30 minutes
- Category: Appetizer
- Method: Baking
- Cuisine: Italian
- Diet: Vegetarian
