Boil the potatoes for 15 minutes from the moment the water comes to a boil, then let them cool in the cooking water.
Sauté the spinach in a pan for a couple of minutes with a knob of butter and a clove of crushed garlic, subsequently removed.|
Peel and grate the potatoes with the large-hole grater, and add salt and pepper.
Spread a sheet of parchment paper on a baking tray; place a ring mold on it (3 inches), grease the bottom of the paper with soft butter, spread an eighth of the grated potatoes inside and press them into a 0.5 inch thick disc.|Slip off the ring and repeat the operation another 7 times forming the rösti.
Unroll the puff pastry dough on a baking tray, leaving it on its baking paper, fold about an inch of the edge inward by pressing it lightly and pinching it about every 1 inch.|
Mix the spinach with egg, the cream, 2 tablespoons of grated Grana Padano cheese, some freshly ground pepper and a grating of nutmeg.
Spread the spinach onto the puff pastry and bake at 390°F.
After 10 minutes, put the rösti in the oven as well.
After another 20 minutes, take out the pie and the rösti.
Let both rest for 5-10 minutes, then transfer the pie to the plate, place the rösti in the center and serve.