Instructions
- Prepare the Pastry: In a large bowl, whisk together the flour and salt. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Add the apple cider vinegar. Gradually add ice water, one tablespoon at a time, mixing until the dough just comes together. Divide the dough in half, flatten into disks, wrap in plastic wrap, and refrigerate for at least 30 minutes.
- Prepare the Filling: Heat the olive oil in a large skillet over medium heat. Add the onions and cook, stirring occasionally, until caramelized and golden brown, about 20-25 minutes. Add the sliced potatoes, thyme, salt, and pepper to the skillet and cook for another 5-7 minutes, stirring occasionally.
- Assemble the Pie: Preheat oven to 375°F (190°C). Roll out one disk of pastry and line a 9-inch pie dish. Pour the potato and onion mixture into the pie crust. Pour the heavy cream over the filling, then sprinkle with nutmeg. Sprinkle the Gruyère cheese evenly over the top. Roll out the second disk of pastry and cover the pie. Crimp the edges to seal. Cut vents in the top crust.
- Bake: Bake for 45-50 minutes, or until the crust is golden brown and the filling is bubbly. Let cool slightly before serving.
