1. Grate potatoes using the large holes of a box grater or a food processor fitted with the grater attachment. Squeeze as much liquid from potatoes as possible, then transfer potatoes to a large bowl. Add three-quarters of scallions, the flour, 1 teaspoon salt, the thyme, and pepper and mix well.
2. Heat oil in a medium nonstick skillet over medium until shimmering. Add potato mixture and use a spatula to press into a firm, even layer that fills bottom of skillet. Cook until bottom is golden brown and top is mostly translucent, 10 to 12 minutes.
3. Gently slide rosti out of skillet and onto a large plate. Cover with a second large plate and, holding sides firmly, invert so that browned side of rosti is now facing up. Carefully slide back into skillet and continue cooking until second side is golden brown, 10 to 12 more minutes. Meanwhile, combine sour cream, apple, remaining scallions, the sugar, and remaining 1/4 teaspoon salt in a small bowl and stir until blended; set aside.
4. Slide rosti onto a cutting board and slice into six wedges. Transfer to a serving plate and serve with apple-scallion sour cream.
Source: Hannaford fresh Magazine, December 2020