In a pot off the heat, place the potatoes (drain before using), milk, salt, thyme and garlic. Season with pepper. Bring to a boil. Simmer over medium heat for 5 minutes or until the potatoes are al dente, swirling the pot a few times to make sure the milk doesn’t stick. Using a slotted spoon, remove the potatoes and place on a large plate.
Add the squash slices to the pot of milk and bring to a boil. Add more milk, as needed, to just cover the squash. Simmer over medium heat until al dente. Drain the squash and place on a large plate. Discard the milk, thyme and garlic.
With the rack in the lowest position, preheat the oven to 400°F (200°C).
On a lightly floured work surface, roll out the larger disc of dough until 1/8 inch (3 mm) thick into a circle that’s at least 13 inches (33 cm) in diameter. Use the dough to line an 8-inch (20 cm) springform pan with removable bottom. Prick the dough all over with a fork. Roll out the smaller disc of dough into an 8-inch (20 cm) circle. Using a small ¾-inch (2 cm) round cookie cutter or pastry tip, make a hole at the centre of the circle of dough to let steam escape as the torte cooks. Set aside.
Egg Wash
In another bowl, combine the egg and milk. Set the egg wash aside.
Assembly
