Make the dough

1. Stir together flour and salt in a large bowl, then add the water, yogurt, 1/2 cup oil, and vinegar. If you have a stand mixer or food processor, it’s best to use these to very briefly knead the dough so that you don’t inadvertently add too much flour. You want a soft, smooth, but pliable dough. If you knead by hand, knead about 4 minutes and resist adding extra flour if at all possible.
2. Oil a large bowl, form the dough into a ball, and roll it around inside the bowl to coat. Cover the bowl with plastic wrap or a damp towel and let dough stand at room temperature 1 hour. This will allow the dough to rest and relax, which will make it far easier to roll out–so it’s definitely worth the wait. (You can keep the dough up to three days in the fridge before using, tightly covered with plastic or in a plastic bag.)

Make the filling

1. Wash the leeks and celery well and drain them, if you haven’t already.
2. Heat 1/4 cup oil in a deep 12-inch heavy skillet or a 5- to 6-quart heavy pot over moderately high heat until hot but not smoking (as you don’t want to brown the vegetables once you add them).
3. Sauté the leeks and celery with 1/2 teaspoon salt, stirring frequently, until softened and translucent, around 10 minutes.
4. Transfer to a bowl and cool about 10 minutes (stirring will promote cooling). Note: you can do this step a day in advance and refrigerate but like the dough bring to room temperature before proceeding.
5. Place your oven rack in the middle position and preheat the oven to 375°F (190°C).
6. Stir the herbs into the leek and celery mixture along with the cheeses, pepper, and remaining 1/2 teaspoon salt. Lightly beat eggs with remaining 3 tablespoons oil in a small bowl, then stir into leek filling. (This is why you want to allow the filling to cool slightly–so as not to cook the eggs or prematurely melt the cheese).

Make the pie

1. Brush baking pan with 2 tablespoons oil.
2. Divide dough in half and flour a work surface. Roll out one half on a floured surface into a rectangle about an 1 1/2 to 2 inches wider on all sides than your pan (i.e. if you have a 17 X 12 inch pan, you want to roll out to about 20 X 15 inches at a minimum, or even a bit bigger). If the dough resists, let it rest about 10 minutes, which will allow the gluten to relax and will result in the dough being more yielding. Fold your rectangle loosely into quarters and transfer to your pan, then unfold dough and fit into the pan, leaving a 1-inch overhang. Spread the filling evenly in pan.
3. Roll out remaining dough on floured surface with floured rolling pin into a rectangle about 1 inch wider on all sides than your pan. Lift dough and drape it over filling, leaving it slightly wrinkled.
4. Roll edge of bottom crust over top to form a rope edge all around pie. Brush top of pie with remaining 2 tablespoons oil (or drizzle it on and use your fingers to spread the oil out over the surface). Score top crust into serving pieces with a sharp knife.
5. Bake the pie until golden brown, 50 to 60 minutes. Allow to cool and serve at room temperature.