Cut the scallions into paper-thin rounds three-quarters of the way up their green sections. Put in a bowl. Pour 4 cups of ice water over them. Cover and refrigerate for an hour.
Prick the eggplant in five to six places with a fork. Now lay it directly over or under a flame. If you are roasting it on top of the stove, keep the heat on low. If you are roasting it under the broiler, keep the eggplant a few inches away from the heat. As one side gets charred, turn the eggplant over slightly, using a pair of tongs to do so. Roast the entire eggplant this way. It should turn very limp by the time it is done.

Peel the eggplant under cold running water and leave it to drain in a colander for 5 minutes. Then mince the flesh.

Put the yogurt in a bowl and beat lightly with a fork or a whisk until smooth and creamy. Add the eggplant, garlic, mint, salt, olive oil, and black pepper. Drain the scallions and pat dry. Add them to the yogurt and mix.

Serve at room temperature or chilled.