1. For the pastry, combine flour and salt in a large bowl. With pastry blender or 2 knives, cut in butter, until mixture resembles coarse crumbs. Add water 1 tablespoon at a time, tossing with a fork, just until mixture begins to hold together. Gather into a ball. Flatten into a thick disk. Wrap well in plastic wrap and refrigerate 30 minutes or overnight.
2. Heat oven to 425 degrees F. On a lightly floured surface, roll out pastry into a 12-inch circle. Fit into a 9-inch tart pan with removable bottom. Fold in edge of pastry; press against side of pan and trim. Prick bottom of pastry shell with fork. Freeze 15 minutes. Line pastry shell with foil; fill with dried beans or rice. Bake 12 minutes; remove foil and beans. Bake 15 to 17 minutes more, until crust is golden. Cool completely on wire rack.
3. Meanwhile, for the filling brush a large cookie sheet with oil. Arrange zucchini slices on prepared sheet in a single layer; sprinkle tops with 1/8 teaspoon salt. Bake 6 minutes; turn slices and bake 6 minutes more, just until softened. Cool on pan.
4. Melt butter in a large skillet over medium heat. Add leeks, thyme leaves and 1/2 teaspoon salt. Cover and cook over medium-low heat until leeks have softened, 5 to 6 minutes.
5. Mix heavy cream, eggs, leeks and pepper in a medium bowl. Pour into crust. Arrange overlapping slices of zucchini on top of cream mixture. Sprinkle top with cheese and thyme sprigs. Bake 25 to 30 minutes, until filling is set and golden on top. Cool on a wire rack 30 minutes. Serve warm or at room temperature.