Grate sweet potato and potato into a bowl. Grate in onion. Add flour, salt, and pepper to bowl and mix to combine. Leave mixture to sit in a colander over a bowl for a few minutes to drain. Remove excess liquid by squeezing mixture with your hands, or by adding to a towel and squeezing.
Heat vegetable oil in a non-stick skillet on medium-high heat. Add mixture a bit at a time until the bottom of the pan is covered. Turn heat down to medium and let cook. When the mixture is golden on the bottom, flip over or turn onto a plate and slide back into the skillet to cook on the other side.
Remove from skillet. Cut into triangles. Serve with creamy Horseradish Dip (see recipe).
For the Horseradish Dip
Add sour cream, mayonnaise, horseradish, Dijon mustard, onion powder, garlic powder, chives, salt and pepper to a bowl and stir to combine.