Placethe yogurt in a cheesecloth or paper towel-lined sieve and set it overa bowl. Grate the cucumber and toss it with 1 tablespoon of koshersalt; place it in another sieve, and set it over another bowl. Placeboth bowls in the refrigerator for 3 to 4 hours so the yogurt andcucumber can drain.
Transfer the thickened yogurt to a large bowl. Squeeze as muchliquid from the cucumber as you can and add the cucumber to the yogurt.Mix in the sour cream, vinegar, lemon juice, olive oil, garlic, dill,1/2 teaspoon salt, and pepper. You can serve it immediately, but Iprefer to allow the tzatziki to sit in the refrigerator for a few hoursfor the flavors to blend. Serve chilled or at room temperature.