Grate the russet potatoes and sweet potatoes on the large holes of a box grater and transfer to a large colander set over a bowl. Grate the onion and add to the colander. Sprinkle with 1 teaspoon salt and 1/2 teaspoon pepper. Firmly squeeze the mixture with your hands to extract as much liquid as possible (blot dry on paper towels if especially wet). Transfer the mixture to a large bowl. Mix in the chives. Add the flour and egg and stir until thoroughly combined.
Using 1/4 cup of the mixture at a time, form tightly packed pancakes about 2 inches wide and transfer to a paper towel-lined baking sheet.
Heat about 1/4 inch of vegetable oil in a large nonstick skillet over medium heat until shimmering. Fry the pancakes in batches until golden brown and crisp, 4 to 5 minutes per side. Transfer to a baking sheet in the oven while cooking the remaining pancakes.
Serve with applesauce, prepared horseradish and/or sour cream.
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