Flamiche aux Poireaux — Picardy Leek Pie

Cut the dried-off leeks into thin half-moons and finely chop the shallot. Melt 35 g butter in a wide skillet, then add the vegetables and salt. Cook gently over low heat with a lid for about 10 minutes, stirring occasionally. Remove the lid and cook for about 10–15 minutes more, until the leeks are completely soft and no liquid pools in the bottom. Do not brown them; lower the heat if necessary. Use all the vegetables with their butter, without draining.

Transfer the leeks to a wide bowl and leave for about 30 minutes, until they cool to room temperature. Stir in the crème fraîche, whole egg, pepper and nutmeg. The filling should be thick and moist, without freely running liquid. Do not mix the egg with hot leeks and do not put warm filling on the pastry.

Start preheating the oven to 200°C, conventional oven without a fan, with a metal baking sheet in the lower part of the oven. Grease a 24 cm metal tart pan, about 3 cm deep, with the remaining 5 g butter. Divide the cold pastry into 280 g and 220 g portions. Roll each portion between two sheets of baking parchment into a round about 3 mm thick, without dusting with flour. Line the bottom and sides of the pan with the larger round, leaving about 1 cm overhanging the rim. The smaller round should cover the filling and meet this edge. Do not knead the pastry; if it becomes very soft, chill briefly before continuing.

Beat the yolk with the water. Spread all the cooled filling over the base, level it and leave the pastry edge clean. Brush this edge lightly with the yolk mixture and cover with the smaller round. Fold the overhanging edge inward and seal it carefully to the lid, pressing lightly with your fingers. Cut a small cross in the center of the lid and gently open the pastry to make a steam vent. Brush the top with a thin layer of yolk mixture, avoiding the hole and the outer pastry edge. Discard any unused brushing mixture.

Refrigerate the assembled pie for about 15 minutes while the oven and baking sheet continue heating. The oven must reach the set 200°C before baking. The hot baking sheet will help cook the base through; transfer the pan onto it carefully using oven mitts.

Set the pan on the hot baking sheet and bake for about 40–50 minutes. The lid should rise in distinct layers and turn golden, the edges should be well baked, and the filling at the vent should be set. If the top browns too quickly, cover it loosely with aluminum foil. Toward the end, gently lift the pastry edge with a flat spatula: the base should be dry and golden. If it is pale and soft, bake for about 5–10 minutes more with foil shielding the top. Do not shorten baking just because the lid has browned.

Set the pan on a rack and leave for about 15 minutes so the filling firms up slightly. Cut into six portions and serve warm, with a crisp lid and a soft, creamy center.