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).