To prepare yogurt: Line a colander or strainer with cheese cloth and set over a bowl. Bowl should support strainer without touching the bottom of the bowl. Put yogurt in the strainer and let drain overnight. Discard liquid and use strained yogurt according to recipe.
Prepare a grill or broiler or heat an oven to 375 degrees F.
Pierce eggplant with a fork. In an oven broiler or on a hot grill, cook eggplant, turning frequently to prevent burning, until skin becomes black and eggplant is soft, about 10 to 15 minutes. Remove eggplant from heat and cool on a wire rack over a baking sheet to catch juices. Eggplants may be baked for 45 minutes for similar results. When eggplant is cooled, peel off skin and coarsely chop. With paddle attachment in an electric mixer, add eggplant, tomatoes, onion and garlic, and mix on medium speed about 1 minute. Stop machine and add yogurt. Continue to mix at medium speed for another minute. With mixer running, slowly add vinegar and lemon juice. Continue mixing while slowly adding olive oil. Thin mixture with seltzer as needed until eggplant salad is the consistency of a loose paste. Add parsley, oregano, and salt and pepper to taste. Refrigerate overnight and serve.