Slice the eggplants in half lengthwise, up to the stem, but not all the way through; eggplants should still be attached and open like a book. Spoon 2/3 of the onion and garlic onto one side of the eggplant halves and press the halves back together.
Lay eggplants on grill and cook 10 to 15 minutes, flipping a few times; eggplants should be blackened and soft.
In the mean time, mix together the rest of the garlic and onion with the yogurt, salt, black pepper, and cayenne pepper.
When the eggplants are finished, let them cool, then peel off the skin and let them cool again. Chop the eggplant into large chunks, saving juices and seeds. Mix eggplant and juices with yogurt. Chill or serve at room temperature; top with mint.