1. Coarsely grate sweet potatoes on the large holes of a box grater (or use a food processor fitted with a shredding blade). Transfer to a large bowl; stir in scallions, eggs, flour, 1 teaspoon coarse salt, and 1/2 teaspoon pepper.

2. Using a packed 1/4-cup measure of potato mixture per pancake, shape into 12 mounds. With hands, form each into a firm ball; transfer to a baking sheet. With palm, flatten balls into 3/4-inch-thick cakes.

3. In a large skillet, heat oil over medium until hot (see note, below). Using a metal spatula with a thin blade, carefully transfer 6 cakes to skillet; flatten to about 1/2 inch thick. Cook until golden brown, 4 to 5 minutes per side. Transfer pancakes to a paper-towel-lined plate; sprinkle with coarse salt. Repeat with remaining cakes. Serve immediately, with sour cream, if desired.