• Fast

Ingredients

1. All-purpose flour, for dusting
2. 1/2 pound frozen puff pastry, thawed
3. 2 tablespoons extra-virgin olive oil
4. 3 medium leeks, white and tender green—halved lengthwise, thinly sliced crosswise and rinsed well
5. 1 teaspoon finely chopped thyme
6. Salt and freshly ground pepper
7. 1/2 pound Gruyère cheese, coarsely shredded (2 cups)
8. 3 ounces thinly sliced prosciutto

Directions

1. Preheat the oven to 475°. On a lightly floured surface, roll out the pastry to a 13-inch square. Fold the corners in and lightly roll the pastry into a rough round. Transfer to a baking sheet; refrigerate. 
2. In a large skillet, heat the olive oil until shimmering. Add the leeks and thyme, season with salt and pepper and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. 
3. Sprinkle half of the cheese over the pastry, leaving a 1-inch border. Spread the leeks over the cheese. Cover with the prosciutto; sprinkle on the remaining cheese. Season with salt and pepper. Fold up the tart edge to form a rim and bake for 20 minutes, until golden and bubbling. Blot any excess fat with a paper towel. Cut the tart into wedges.