Slice leeks thinly using the white and light green parts and discarding the tougher, dark green sections.
Wash sliced leeks in cool water and drain.
Sauté leeks in oil until tender, then drain in colander.
Steam or boil potatoes until cooked through and tender, then drain thoroughly.
Beat eggs with a whisk. Add yogurt, milk, and spices and whisk until thoroughly blended.
Spread leeks, potatoes and cheeses evenly in a well-oiled dish or pan.
Add the egg mixture evenly to the pan. Mix the ingredients together gently.
Cover with foil or lid and bake quiche for about an hour. It is done when the top of the quiche is firm and springy and an inserted knife comes out cleanly (melted cheese is okay) and any liquid runs clear. If the top is not brown enough, turn the oven temp down by 25 degrees and continue to bake uncovered for the last 10 minutes.
Let quiche sit for 10 mins to set before serving.