The barely-there binder in the batter makes for incredibly lacy, crispy fritters. Be sure to cook them gently over medium heat so they cook through and crisp up without burning.
Whisk together yogurt, tahini, lemon juice, garlic, pepper, and 1/2 teaspoon salt in a small bowl. Set aside.
Place a colander over a large bowl; add sweet potatoes, turnips, and remaining 2 teaspoons salt; toss to combine. Let mixture stand 10 minutes. Transfer mixture to a clean kitchen towel; gather up edges, and squeeze out excess liquid. Transfer sweet potato mixture to a medium bowl.
Heat olive oil in a large cast-iron skillet over medium. Add leeks and a pinch of salt, and cook, stirring occasionally, until leeks are soft and starting to brown, about 5 minutes. Remove from heat. Add cooked leeks, eggs, flour, cumin, turmeric, and ginger to sweet potato mixture, and stir until thoroughly combined.
Wipe out skillet, and add a 1/8-inch layer of vegetable oil. Heat over medium until shimmering. Working in batches, spoon 2 tablespoons of batter for each fritter into skillet about 2 inches apart, and flatten slightly with a spatula. Cook until golden and crisp, 2 to 3 minutes per side. Transfer fritters to paper towels to drain; sprinkle with salt to taste. Add more oil to skillet between batches as needed. Serve with lemon-tahini sauce and kale.