Preheat your oven to 375°F (190°C).

Slice the leeks into thin rounds and wash them thoroughly in a colander to remove any grit.

Melt the butter in a large skillet over medium-low heat. Add the leeks and cook gently for 10-15 minutes until very soft but not browned.

While the leeks cook, whisk together the eggs, heavy cream, nutmeg, salt, and pepper in a medium bowl.

Roll out the puff pastry and press it into a 9-inch tart pan or pie dish, trimming any excess from the edges.

Spread the cooked leeks evenly over the bottom of the pastry crust.

Sprinkle the grated Gruyère cheese over the leeks.

Pour the egg and cream mixture carefully over the filling.

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

Let the tart cool for 10 minutes before slicing and serving.