Trim the leek and rinse thoroughly. Drain and slice into 0.5 cm rounds. Grease the bottom and side of a 30-cm quiche dish with little butter and cover with the puff pastry.
Heat the oil in a large frying pan over medium-high heat and sauté the bacon for a few minutes. Add the leek, season with a little salt and pepper. Cook for about 10 minutes, stirring occasionally until nicely wilted. Let cool for 5 minutes and spread over the puff pastry.
Whisk the eggs with the cream, ½ teaspoon salt and ¼ teaspoon freshly ground nutmeg. Pour the mixture over the leek and cover with cheese.
Heat oven at 200 °C/ 400°F and bake for about 40 minutes. Let stand a few minutes before serving.