Trim the very ends of the eggplant and cut it crosswise into 1-inch-thick slices. Put the slices in a single layer in a large platter or lasagna-type dish. Sprinkle the salt over both sides, rubbing it in well. Set aside for 1 hour. Meanwhile, make the tomato sauce. Put the oil a large, nonstick frying pan set over medium-high heat. When hot, add onion. Stir-fry for 2 to 3 minutes, or until the onion pieces begin to brown at the edges. Put in the garlic. Stir for a few seconds. Then put in the tomatoes and their liquid as well as all the remaining ingredients for the tomato sauce. Stir to mix. Cover, turn the heat to low, and cook gently for 10 minutes. Set aside in a warm place. Yogurt sauce: Put the yogurt in a small bowl and beat lightly with a fork. Just before you eat, put oil to a depth of 2 to 3 inches for deep-frying in a wok or deep-fryer and set over medium heat. Take the eggplant slices from the platter and dry them off well with paper towels. When the oil is hot, drop in as many slices as the utensil will hold easily and fry, turning now and then, for 6 to 7 minutes, or until both sides are a medium brown color. Drain well on paper towels. To serve, arrange the eggplant slices in a single layer on a large platter. Top each slice with a dollop of the tomato sauce and then with a tablespoon of the yogurt. Garnish with the mint sprigs or leaves. Serve immediately.