How to Make It
Preheat the oven to 400°. In a colander, rinse the julienned potatoes until the water runs clear; drain well and pat very dry. In a large bowl, toss the potatoes with 2 teaspoons of salt and 1/2 teaspoon of pepper.
In a 10-inch ovenproof skillet, heat the canola oil until shimmering. Remove the pan from the heat and add half of the potatoes. Return the skillet to the heat and, using a spatula, spread the potatoes in an even layer. Scatter the 1 cup of thinly sliced scallions evenly over the potatoes, then scatter the remaining potatoes on top. Using the spatula, shape the potatoes into a neat cake.
Cook the potatoes over moderately high heat until well browned on the bottom, 9 minutes. Using a metal spatula, flip the cake, then cook over moderately high heat until lightly browned on the bottom, 5 minutes. Transfer the skillet to the oven and bake for 12 minutes, until cooked through. Slide the hash brown onto paper towels to drain. Transfer to a plate and season with salt and pepper. Garnish with sliced scallion greens and serve with sour cream.