Preheat oven to 200 degrees. Clean, wash and drain the leek, cut into thick rings. Fry the bacon in 1 tablespoon of fat. Steam the leek in it for 5-8 minutes. Let cool down.
Mix the grated cheese with the cream, eggs and flour. Season with salt, pepper and nutmeg. Stir in the leek. Place the puff pastry with baking paper in a springform pan (26 cm diameter), pull up the edge and press firmly all around.
Pour the leek mixture into the mold, smooth it out and distribute the remaining flakes of butter on it. Bake the leek cake for 45 minutes at 200 degrees and serve fresh from the oven.