Heat oven to 500F. Toss eggplant cubes with 1/3-cup olive oil and season with salt and pepper. Spread in a single layer on large baking tray and roast 10 minutes. Continue roasting until eggplant browns, another 10-15 minutes, stirring every few minutes. Heat remaining oil in a large skillet and saute onions 10 minutes. Add tomatoes, olives and capers. Reduce heat to a simmer, cover and cook about 15 minutes. Add eggplant and celery to pan and continue cooking, covered, for about 8 minutes. Remove cover, turn heat up to high and stir in vinegar and sugar. Cook until vinegar evaporates and all flavors meld, about 3 more minutes. Season to taste with salt and pepper. Serve at any temperature with pasta, bread, polenta, etc