Preheat your oven to 400°F (200°C) and lightly grease a 9-inch tart pan with a removable bottom.

Roll out the thawed puff pastry on a lightly floured surface, transfer it to the prepared tart pan, press gently into the edges, trim the excess dough, and prick the bottom with a fork.

In a large skillet over medium heat, cook the chopped bacon until crispy, about 6 to 8 minutes, then remove with a slotted spoon and set aside, reserving 1 tablespoon of bacon fat in the skillet.

Add the butter and sliced leeks to the skillet, cooking over medium-low heat for 12 to 15 minutes until the leeks are very soft and translucent but not browned.

In a medium bowl, whisk together the eggs, heavy cream, crème fraîche, ground nutmeg, salt, and black pepper until completely smooth.

Spread the cooked leeks and crispy bacon evenly over the bottom of the pastry shell, then sprinkle the shredded Gruyère cheese on top.

Pour the egg and cream mixture slowly over the filling, ensuring it distributes evenly without overflowing the pastry edges.

Bake for 30 to 35 minutes until the pastry is golden brown and the custard center is puffed and just set with a slight jiggle.

Let the flamiche cool for 10 minutes in the pan before slicing, and serve warm alongside a crisp, dressed green salad.