How to Make It
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.