How to Make It
Preheat the oven to 425°. Toss the eggplant with 4 tablespoons olive oil in a large bowl, and season with salt and pepper. Arrange the eggplant on a baking sheet in 1 layer and roast until browned and tender, flipping halfway through, about 25 minutes. Let cool.
Meanwhile, cook the celery in a small pot of boiling water until crisp-tender, about 3 minutes. Drain and rinse under cold running water until cooled. Drain well, and set aside.
Heat the remaining 2 tablespoons olive oil in a large skillet over moderately high heat. Add the onion and cook, stirring occasionally, until just golden, about 5 minutes. Add the olives, capers, reserved celery, tomato sauce, sugar, vinegar and salt to taste. Gently stir in the eggplant, being careful not to break up the pieces. Simmer for 2 to 3 minutes, then transfer to a large bowl or platter and cool.
Garnish the caponata with the chopped parsley, and serve cold or at room temperature.