Rinse leeks well, and drain; set aside.
Melt butter with oil in a medium sauté pan over medium heat. Add leeks and sprinkle with salt; cook, stirring occasionally, 5 minutes, cook until translucent and soft, add thyme. Leeks can be refrigerated in an airtight container up to 1 day; bring to room temperature before assembling tart.
Cut or roll out pastry to a 6-by-14-inch rectangle; place on a parchment-lined baking sheet (reserve remaining pastry for another use). Score a 3/4-inch border. Brush with egg wash; sprinkle with Parmesan. Refrigerate 30 minutes.
Preheat oven to 375°. Bake pastry until light brown; remove from oven, and press center area with spatula. Using an offset spatula or a spoon, smear the pastry with the cheese within the border, add leeks and olives, and bake until crust is golden brown, about 10 minutes. If bottom is soft, bake 3 to 5 minutes more.
Remove tart from oven place tart onto a wire rack; let cool slightly. Cut into pieces; serve warm or at room temperature (tart can stand at room temperature up to 1 hour).