Preheat oven to 375˚F. In a medium-size bowl, toss leeks with oil and season with salt and pepper to taste. Set aside.
In a separate medium-size bowl, stir together chèvre, cream cheese, milk, 2 egg yolks, and thyme. Season to taste with salt and pepper and set aside.
In a separate small bowl, beat remaining egg yolk with 1 tablespoon water and set aside.
Roll out Savory Piecrust dough into a round 12 inches wide and ⅛ inch thick. Spread chèvre mixture over dough, leaving a 2-inch border. Top with leeks.
Fold edges of dough over filling. Brush with egg wash. Bake until crust turns golden brown, about 50 minutes. Let cool on a wire rack.
In a medium-size bowl, whisk together f lour and salt until well combined. Sprinkle butter over f lour mixture and use your fingers to work it in (rub your thumb against your fingertips, smearing the butter as you do). Stop when the mixture looks like cornmeal, with some pea-sized bits of butter remaining.
Sprinkle ice water on top and stir with a fork until dough begins to come together. If needed, add more ice water, a tablespoon at a time. Turn out onto a lightly floured surface and knead three times, or just enough to make a cohesive dough—don’t overmix! Gather into a ball; then press into a disk and wrap in plastic wrap. Refrigerate 30 minutes. Yield: dough for 1 crust