Preheat the oven to 200 °F. Grate onion and sweet potatoes using a food processor fitted with a medium grate, or on the large holes of a box grater. Combine in a medium bowl.

Place half of the onion and potato mixture on a clean kitchen towel. Roll the towel around the mixture and wring the towel to draw out excess moisture. Unroll the towel and transfer the mixture to a large bowl. Repeat with the remaining onion and potatoes

In a small bowl, whisk together the white rice flour, salt, and pepper. Add to the potato mixture and stir to combine. Add the eggs and stir to combine. Line a rimmed baking sheet with several layers of paper towels and set it near the stove but safely away from the burner. Heat 1/4 inch of oil in an 8-inch cast-iron skillet over medium-high heat until shimmering.

Working in batches, drop the potato mixture into the hot oil by scant 1/4-cups. (The mixture should sizzle when it hits the oil.) Using two forks, flatten each latke a little in the pan. (You almost “pull” the latkes apart to flatten.)

Fry until deep golden brown, 2 to 3 minutes. Flip with a pancake flipper or spatula and fry an additional 2 to 3 minutes. Transfer to a paper towel–lined baking sheet and keep warm in oven while you cook remaining latkes. Serve with sour cream and apple sauce.