Grate the potatoes and onion on the large holes of a box grater. Transfer to a fine-mesh strainer or clean kitchen towel and squeeze out as much liquid as possible.
Grate the sweet potato and carrots and finely chop the cilantro. Add to a large bowl with the eggs, flour, salt and pepper and mix until well combined.
Heat the oil in a large skillet over medium heat. Form the vegetable mixture into fritters, squeezing out any excess liquid as you shape them. Fry in batches until golden brown and crisp, about 2 minutes per side.
Transfer the fritters to a paper towel-lined plate to drain.
Make the tahini: Whisk the tahini, lemon juice, garlic and salt in a small bowl. Gradually whisk in the cold water until smooth and creamy. If the sauce is too thick, whisk in more water, 1 tablespoon at a time.
Serve the fritters warm with the tahini sauce.