Heat the oven to 200°F. Line an ovenproof plate or baking sheet with paper towels. Peel and grate the sweet potatoes and squeeze or press them dry if necessary; you want 3 packed cups; save the rest for later.
Combine the grated sweet potato, flour, cornmeal, onion, egg, and a sprinkle of salt and pepper and mix well with a fork. If the mixture looks too liquid, add more flour, 1 tablespoon at a time. (You can make the batter ahead of time and refrigerate for up to a couple of hours before cooking.)
Put 2 inches of oil in a large pot over medium heat. When the oil is hot, carefully drop spoonfuls of the sweet potato mixture into the pot. (Work in batches to avoid crowding the pot.)
Cook, turning them with tongs or a slotted spoon as necessary so they brown on all sides, until they’re cooked through, 5 to 7 minutes. Transfer the finished fritters to the paper towel–lined plate and put it in the oven to keep warm while you make the rest. Serve hot or at room temperature, with more salt and pepper if you like.