Preheat oven to 500 Fahrenheit.

Grate the sweet potatoes, white potatoes and onions in the food processor or by hand. Drain white potatoes in a wire mesh colander and gently squeeze between paper towels. Squeeze the sweet potato and onion before combining with the flour mixture.

In a large bowl, combine flour, salt, and pepper. Beat in eggs. Stir in the potatoes and onion.

Heat 1 tablespoon oil in a large, heavy skillet over medium heat. Scoop mixture with a tablespoon or a soup spoon, thoroughly squeezing each spoonful to remove excess moisture. Slide mixture into the pan, gently flattening each pancake with a spatula. Cook about 4 minutes on each side, or until brown. Remove and drain on paper bags. Repeat in two more batches. You can flash freeze them on baking sheets without the paper bags at this point if you will serve them later. To serve right away or to cook from frozen, transfer all of the pancakes to two baking sheets lined with paper bags. Finish cooking them in the oven until crisp, about 4 minutes on each side. Serve immediately with sour cream, applesauce, and my personal favorite, strawberry raspberry preserves.