Preheat your oven to 375°F (190°C).

Clean the sliced leeks thoroughly in a bowl of cold water to remove any grit, then drain well.

In a large skillet over medium heat, cook the bacon lardons until crisp, about 5 minutes. Remove the bacon with a slotted spoon and set aside.

Add the butter to the bacon fat in the skillet. Add the leeks and cook gently for 10-15 minutes until very soft but not browned. Season lightly with salt and pepper.

While the leeks cool slightly, roll out the puff pastry and press it into a 9-inch tart pan or pie dish. Prick the base with a fork.

In a mixing bowl, whisk together the eggs, heavy cream, nutmeg, and half of the grated Gruyère cheese.

Spread the cooked leeks and bacon evenly over the pastry base.

Pour the egg and cream mixture over the filling and top with the remaining cheese.

Bake for 35-40 minutes, or until the pastry is golden brown and the filling is set.

Let the flamiche cool for 10 minutes before slicing and serving warm.