In the bowl of a food processor or using a box grater, shred the sweet potato, turnip, parsnips, and onion. Transfer to a cheesecloth or kitchen towel lined colander, and wring out the moisture over a bowl.
Let it sit for about 10 minutes, then drain any excess liquid, leaving any starch that may be left at the bottom of the bowl.
Transfer the shredded potato mixture into the bowl and add the flour, eggs, salt, and pepper. Use your hands to mix.
Preheat the oven to 175 degrees F and line a shallow dish or baking pan with paper towels. Heat a skillet (a heavy cast iron or non-stick pan works well) over medium high heat with about 1/4 cup of oil. Drop about two tablespoons of the sweet potato mixture into the pan and use a fork to spread into about 3-inch pancakes. Let it crisp a few minutes per side and transfer to the prepared baking dish. Sprinkle with a little salt if desired.
Keep latkes warm in the oven until serving. But they''re best eaten right away and served with a dollop of sour cream.