Parbake the crust: Preheat the oven to 350°F. Remove the dough from the refrigerator and place it on a well-floured surface. Working from the center, roll the dough into a 12-inch circle, then transfer it to an ungreased 9″ pie dish. With a fork, pierce rows of holes in the bottom and sides of the dough, about 1 inch apart. Bake until the crust is lightly golden, 10 to 15 minutes. Set aside to cool.

Prepare the filling: Heat the olive oil in a large skillet over medium-high heat. Add the leeks and sauté for 5 minutes, until tender. Add the spinach and sauté for 2 minutes. Remove from heat and set aside.

In a large bowl, whisk together the eggs, egg whites, milk, Greek yogurt, salt, and pepper. Set aside.

Spoon the spinach and leek mixture into the bottom of the crust, then sprinkle the feta over the top.

Gently pour in the egg mixture.

Bake the quiche for 45 to 55 minutes, until eggs are set in the center and puffed slightly and the crust is golden brown. Let the quiche cool on a wire rack for 10 minutes. Slice and serve warm or at room temperature.