In a mixing bowl, add all-purpose flour, water, salt, butter and an egg. Mix the ingredients with an electric mixer or with your hands until you get a non-sticky dough.
Wrap the dough and allow it to rest for at least 30 minutes in the refrigerator.
Grease a spring form pan with some butter/margarine and dust all-purpose flour over it, to prevent the leek pie from sticking to it.
In another mixing bowl, add eggs, milk, crème fraîche and shredded cheese. Mix the ingredients with an electric mixer or with your hands. Set it aside, we will come back to it soon.
Remove the top (dark green part) and bottom part of the leek. We would only need the green and white part of the leek. Cut the leek into two half moons along its length and wash them thoroughly. Thereafter, slice the leek halves into tiny pieces (semicircles).
Cut the onions into tiny cubes.
In a frying pan, heat up 2 tablespoons of cooking oil over medium-high heat. After which, add in bacon and cook the cubes until they turn crispy.
Add leek and onions to the bacon cubes. Sauté leek and onions for several minutes until they reduce in size, and cook until the leek turns soft. Set aside the frying pan, we will need it later on.
Preheat baking oven to 200 °C.
Take your pie dough out of the refrigerator and dust some flour over the working space. Roll out your dough into a circle shape (like a pizza) with a rolling pin. It should be about 32 cm in the diameter.
Place the leek pie dough into the springform cake pan. About 4 cm of dough in height at the sides. Make the height of the dough is even across the pie. Gently press the dough against the side of the pan and stick toothpicks into the side of the dough to support it. This is because the side of the pie will fall and shrink once it is prebaked. For more details, please watch the tutorial.
Gently poke holes with a fork to the bottom of the pie. Prebake the leek pie for 10 minutes.
After baking the pie for 10 minutes, take the leek pie out of the baking oven. It is possible that some toothpicks did not hold up the sides of the pie, causing it to fall. Simply adjust the toothpicks (be careful, the pan is hot!) accordingly.
Add the sautéed leek and bacon from the frying pan (step 6) to the bottom of the leek pie. Thereafter, pour the egg mixture over the leek and bacon cubes (step 7). Distribute shredded cheese evenly on the surface of the leek cake.
Remove all toothpicks and bake the German leek cake for another 30 to 35 minutes, or until the cheesy surface turned slightly brown. Adjust the timing according to your baking oven’s specification.
Remove the pie from the oven and allow it to cool down for several minutes before trying to remove it from the springform cake pan.
Serve it warm with your favourite salad! It can be enjoyed cold as well!