Cut unpeeled eggplant into small cubes. In a large skillet, heat 1/2 cup olive oil over moderately high heat. Sauté eggplant, turning and stirring until nicely browned, about 10 minutes.

Add onion, celery and green pepper. Cook and stir until vegetables are crisp-tender, adding more oil if necessary. Stir in tomato purée, olives, vinegar, sugar and salt. Simmer uncovered 10 minutes, stirring occasionally.

Remove from heat. Cool and refrigerate several hours or overnight. May be refrigerated up to 1 week, or frozen. Makes about 4 cups.