Heat the butter in a large frying pan and fry the potatoes with the onion for 5-6 minutes on a not too high heat. Stir often. The potatoes must be pliable and cooked for one third. Stir in the salt, spices and herbs. Put everything in a colander to drain and cool.

Roll two-thirds of the dough into a 3 mm thick disc. Line a lightly greased pie ring of 22 cm diameter and put it in the fridge for 20 minutes.

Heat the oven to 180 ° C and prick the dough. Divide the potatoes.

Roll out the rest of the dough into a 23-24 cm slice. Brush the edge of the pie base with egg. Wrap the second disc loosely around the rolling pin and roll it over the potatoes. Remove the excess dough and squeeze the 2-3 mm in the edges to seal them; it must be a nice edge. Roll out the trimmings and cut leaves. Brush the top of the dough with egg and place the leaves on it. Brush also with egg and draw on it with the tip of a knife 'veins'. Cut a 'chimney' in the middle and insert a funnel of aluminum foil to allow the steam to escape.

Put the pie in the oven for 45 minutes; check the cooking time by inserting a thin pin through the chimney - the potatoes should be easy to pierce. Put the pie on a rack and carefully remove the ring. Bring the cream to a boil in a saucepan, let it bubble for 2-3 minutes and stir in salt and pepper. Pour through the chimney with small bits at the same time and add the next drizzle only when the previous one has been absorbed (in total about 20 minutes). Cut the pie on the table warm.