Preheat the oven to 350°. Heat the olive oil in a large skillet over medium-high heat. Add the leeks and the bell pepper and sauce two minutes. Remove from heat and set aside.
In a large bowl, whisk together the eggs, egg whites, milk, yogurt, salt and pepper. Set aside.
Transfer the pie plate with the crust to a rimmed baking sheet. 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 on the baking sheet for 45-55 minutes, until the eggs are set in the center and puffed slightly and the phyllo is golden brown. Let the quiche cool on a wire rack for 10 minutes. Slice and serve warm or at room temperature.
Enjoy!