How to Make It
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.
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.
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.