- For the pastry, combine the flour and salt in a large bowl. With a pastry blender or 2 knives, cut in the butter, until the mixture resembles coarse crumbs. Add the water, 1 tablespoon at a time, tossing with a fork, just until the mixture begins to hold together. Gather into a ball, then flatten into a thick disk. Wrap in plastic wrap and refrigerate for 30 minutes or overnight.
- Heat the oven to 220°C. On a lightly floured surface, roll out pastry into a 12-inch circle. Fit into a 22cm tart pan or mini pans with a removable bottom. Fold in edge of pastry; press against side of the pan and trim. Prick the bottom of the pastry shell with a fork. Freeze for 15 minutes. Line pastry shell with foil and fill with dried beans or rice. Bake for 12 minutes; remove foil and beans. Bake 15 to 17 minutes more, until crust is golden. Cool completely on wire rack.
- Meanwhile, for the filling, brush a large cookie sheet with oil. Arrange the baby marrow slices on the 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.
- Melt butter in a large pan over medium heat. Add the leeks, thyme leaves and 1⁄2 teaspoon salt. Cover and cook over medium-low heat until leeks have softened, 5 to 6 minutes. Mix the 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.