Preheat the oven to 250 degrees F. Line a baking sheet with paper towels.
Grate the 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 mixing bowl. Add the kale, chives and dill and toss to combine. Add the flour and egg and stir until thoroughly combined.
Working with 1/4 cup of the mixture at a time, form tightly packed pancakes about 2 inches wide and transfer to the prepared baking sheet.
Heat 1/4 inch of oil in a large nonstick skillet over medium heat until shimmering. Working in batches, fry the pancakes until golden brown and crisp, 4 to 5 minutes per side. Transfer to a baking sheet and put in the oven to keep warm while cooking the remaining pancakes.
Serve with sour cream.
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