Ingredients

• 4 cups all-purpose flour plus additional for dusting
• 2 teaspoons salt
• 3/4 cup water
• 1/2 cup whole-milk yogurt (preferably Greek)
• 3/4 cup extra-virgin olive oil
• 1 tablespoon red-wine vinegar
• 6 lb leeks (white and pale green parts only), finely chopped (about 10 cups)
• 1/4 cup plus 3 tablespoons extra-virgin olive oil
• 5 large celery ribs, finely chopped (about 3 cups)
• 1 teaspoon salt
• 1 cup chopped fresh mint
• 2/3 cup chopped fresh flat-leaf parsley
• 1/2 cup chopped fresh dill
• 1/4 lb Greek feta (preferably Mt. Vikos or Dodoni brand), crumbled (1 cup)
• 1 1/2 oz finely grated Kefalotyri or Parmigiano-Reggiano cheese (1/2 cup; see Tips, page 167)
• 1/4 teaspoon black pepper
• 3 large eggs
• Special equipment: a 17- by 12-inch shallow heavy baking pan (1 inch deep)

Instructions

1. Stir together flour and salt in a large bowl, then make a well in center and add water, yogurt, 1/2 cup oil, and vinegar. Stir until a soft dough forms, then knead on a work surface (do not dust with flour) until smooth and pliable, about 4 minutes. Form dough into a ball and put in an oiled bowl, then turn to coat. Cover bowl with plastic wrap and let dough stand at room temperature 1 hour.
2. Make filling: Wash leeks in a bowl of cold water, agitating them, then lift out and drain well in a colander. 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, then sauté leeks and celery with 1/2 teaspoon salt, stirring frequently, until softened and translucent, 10 to 12 minutes. Transfer to a bowl and cool, stirring occasionally, 10 minutes. Put oven rack in middle position and preheat oven to 375°F. Stir herbs into leek mixture along with 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.
3. Wash leeks in a bowl of cold water, agitating them, then lift out and drain well in a colander.
4. 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, then sauté leeks and celery with 1/2 teaspoon salt, stirring frequently, until softened and translucent, 10 to 12 minutes. Transfer to a bowl and cool, stirring occasionally, 10 minutes.
5. Put oven rack in middle position and preheat oven to 375°F.
6. Stir herbs into leek mixture along with 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.
7. Assemble pie: Brush baking pan with 2 tablespoons oil. Divide dough in half. Roll out 1 half on a floured surface with a floured rolling pin into a 19- by 14-inch rectangle. Fold rectangle loosely into quarters and transfer to baking pan, then unfold dough and fit into pan, leaving a 1-inch overhang. Spread filling evenly in pan. Roll out remaining dough on floured surface with floured rolling pin into a 20- by 15-inch rectangle. Lift dough and drape it over filling, leaving it slightly wrinkled. Roll edge of bottom crust over top to form a rope edge all around pie. Brush top of pie with remaining 2 tablespoons oil. Score top crust into serving pieces with a sharp knife (mark crust but do not cut through it). Bake pie until golden brown, 50 to 60 minutes. Cool to room temperature in pan on a rack, about 1 hour.
8. Brush baking pan with 2 tablespoons oil.
9. Divide dough in half. Roll out 1 half on a floured surface with a floured rolling pin into a 19- by 14-inch rectangle. Fold rectangle loosely into quarters and transfer to baking pan, then unfold dough and fit into pan, leaving a 1-inch overhang. Spread filling evenly in pan.
10. Roll out remaining dough on floured surface with floured rolling pin into a 20- by 15-inch rectangle. Lift dough and drape it over filling, leaving it slightly wrinkled. Roll edge of bottom crust over top to form a rope edge all around pie. Brush top of pie with remaining 2 tablespoons oil. Score top crust into serving pieces with a sharp knife (mark crust but do not cut through it).
11. Bake pie until golden brown, 50 to 60 minutes. Cool to room temperature in pan on a rack, about 1 hour.