Boil potato slices for 5 minutes until tender, then drain and rinse with cold water.
Sauté sliced onions in 25g butter for 5 minutes until soft and preheat oven to 200°C.
Pulse flour, butter, and cheese in a food processor, then add egg and water to form dough.
Knead dough into a ball and set aside 250g for the lid.
Roll out larger pastry portion to 4mm thickness and line a 23cm pie dish.
Layer potatoes, onions, and butter cubes into the crust, seasoning each layer.
Roll out the pastry lid, seal the edges, glaze with egg, and pierce the center.
Bake the pie for 45 minutes until golden-brown and potatoes are soft.
Soak dried mushrooms in 500ml boiled water with bouillon for 20 minutes.
Sauté chestnut mushrooms in oil and butter for 6 minutes until browned.
Stir in flour, then gradually whisk in the reserved mushroom soaking liquid.
Add chopped soaked mushrooms and chives, simmering the gravy until thickened.
Melt butter in a pan and add spinach with nutmeg and seasoning.
Cover and cook for 2 minutes until the spinach is wilted.
Serve the hot pie accompanied by the mushroom gravy and spinach.