Preheat the oven to 425 degrees . Place the potato in a large saucepan with enough salted water to cover by 1 inch. Bring to a boil, lower the heat and simmer until tender, about 8 minutes. Drain, then transfer to a bowl. Reserve the saucepan.
Using a fork, prick the bottom of the piecrust. Transfer to a baking sheet and bake for 10 minutes.
In the saucepan, melt the butter over medium heat. Add the mushrooms and scallions and cook until softened, about 7 minutes. Stir into the potato, along with the pumpkin puree, eggs and sour cream; season with salt and pepper. Spoon into the piecrust, sprinkle with the nuts and bake until set, about 20 minutes.