Add all the yogurt sauce ingredients in a small bowl. Cover the bowl and refrigerate it while you prepare the fritters.
In a large bowl, mix the leeks, kohlrabi, flour, cheese, salt, black pepper and lemon zest. Beat the eggs in a small bowl and mix it in with the vegetables.
Heat about 1 1/2 tablespoons of oil in a large pan over medium heat. Scoop 2 tablespoons of the vegetables and transfer it to the pan. Add 5 more fritters to the pan and cook for about 4 to 5 minutes, flipping halfway. Transfer cooked fritters to a plate lined with paper towels. Repeat for the remaining fritters. If the pan looks dry, add another tablespoon of olive oil to the pan.
Serve fritters with sumac yogurt sauce.