Preheat the oven to 180°C. Butter and flour in a pan. Peel and slice the potatoes. Cut the Saint-Nectaire into slices, without the rind. Using a rolling pin, shape half the pastry to fit the pan. Arrange in the pan. Brush the sides with a little water. Fill with a layer of potatoes.
Cover the potatoes with slices of Saint-Nectaire. Sprinkle with cumin. Finish with a layer of potatoes.
Season the cream with salt and pepper. Pour it over the potatoes. Roll the rest of the pastry to the size of the pan. Arrange it over the filling. Seal the edges of the two layers of pastry with your fingertips, pinching them gently together.
In the center of the pastry, cut out a small circle around half an inch in diameter to create a "chimney" through which the steam can escape. Whisk the egg in a bowl. Brush the pie with the beaten egg. Bake the pie for 45 minutes. Serve it straight from the oven with a crunchy salad.