Boil the potatoes until tender in salted water.
Brush a 23 cm diameter springform pan with melted butter. Put the boiled potatoes in the mold and mash them well into the bottom and up the edges. Bake for 15 min.
Rinse and cut the leek into thin slices. Pour boiling water over and leave the leeks in the water for 3 minutes so they can be blanched. Pour off the water and dab the leeks with kitchen paper so they are not too wet.
Fry bacon cubes on a pan. Add onions just before the bacon is completely crispy. Pour it all onto a plate with kitchen paper to absorb the grease.
Whisk eggs, creme fraiche, grated cheese, salt and pepper together.
Spread the leek and bacon/onion mixture on the potato base and pour the egg mixture on top. Bake the pie for approx. 20 minutes at 200 C fan.
Garnish with chives. Carefully remove the springform pan and serve the pie.