Heat the oil in a medium to large frying pan over medium heat. Cook the bacon for 5 to 7 minutes until browned and crispy. Pour all but 1 tablespoon of oil from the pan and discard when solid. Add the leek and cook, stirring occasionally for 10 minutes until softened; let cool.

Preheat the oven to 375°F (190°C).

Whip cream, eggs, nutmeg and a pinch of salt and pepper together in a medium bowl; set aside.

Grease a 10-inch (25cm) tart pan with a removeable base. Layer a tart pan with a sheet of pastry with the pastry over hanging the side. Place another pastry sheet over the first making a cross. Brush that with butter. Continue layering the pastry sheets, working around the pan and brushing with butter until all the pastry is used. Gently press the pastry around the base of the pan where the side meets the base. Working your way around the pan, fold up the overhanging pastry. Make a few small slits in the pastry to allow the steam to escape. Place the pan on a baking sheet. Bake on the bottom rack of the oven for 15 minutes until a light golden color. Leave the pan on the baking sheet. If the pastry rises very gently press down with some paper towel.

Scatter half of the cheese over the pastry base, then the leek and bacon mixture. Slowly pour in the cream mixture. Scatter the remaining cheese over the top. Bake for 35 to 40 minutes or until the filling is set and the pastry is golden. Let cool to warm or room temperature before cutting into slices. Serve slices with mixed greens.