Preheat oven to 450 degrees F.
Put pie crust in a pie pan (decorate edges if desired), prick with a fork and put in fridge while preparing other ingredients.
Slice leeks in half and thinly slice white part and some of the greens. You want about 4 cups chopped.
Heat olive oil in a skillet over medium heat. Cook leeks until whites become translucent, 5-7 minutes.
In a small saucepan combine 1 egg yolk, 3 eggs and yogurt with a whisk. Heat until warm, do not boil. Add salt and pepper to taste.
Add yogurt mixture and ham to the leeks and stir until combined.
Brush the pie crust with slightly beaten egg white. Add the filling and smooth.
Bake for 10 minutes at 450 degrees. Immediately lower the temperature to 300 and bake until the pie crust is golden brown and the center is set. 10-20 minutes.
Serve immediately or let chill and serve cold.