Preheat oven to 375°
Dice bacon and saute on medium high heat until just short of crisp. Remove from pan to drain and cool, reserving about 2 tablespoons of bacon grease in pan.
Split and wash leeks to remove grit. Dice white and green parts and add to pan in which you have reserved a couple tablespoons of the bacon grease. Add thyme leaves and salt and white pepper to taste. Saute over medium heat until leeks are just starting to become translucent. Remove leek mixture from pan and cool.
Shred gruyere and swiss cheeses in food processor or box grater
Whisk eggs and egg yolk in bowl, add heavy cream and milk, along with cheeses, cooled leek mixture and cooled bacon, mix thoroughly. Pour into prebaked pie shell. Place on cookie sheet in center of oven.
Bake at 375° for about 30 minutes until center is set and top is golden brown.
Let cool and serve at room temperature.