Preheat your oven to 400°F (200°C). Roll out the thawed puff pastry sheet on a lightly floured surface to fit a 9-inch tart pan, press the dough gently into the bottom and sides, then prick the base all over with a fork.

Melt the unsalted butter in a large skillet over medium heat. Add the sliced leeks and sauté for 12 to 15 minutes, stirring occasionally, until they are very soft and translucent but not browned.

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

Spread the cooked leeks evenly over the bottom of the pastry crust, then sprinkle the grated Gruyère cheese uniformly over the top of the leeks.

Carefully pour the custard mixture over the leeks and cheese, ensuring it fills all the gaps. Bake for 30 to 35 minutes until the filling is puffed and golden brown, and the pastry is cooked through.

Let the flamiche cool in the pan for 10 minutes on a wire rack before removing the rim. Slice into warm wedges and serve alongside a crisp green salad.