2. In a skillet, heat 2 tbsp of the oil over medium-high heat for 30 seconds. Add eggplant, in batches, and cook, stirring and tossing, until it begins to brown, about 3 minutes per batch. Transfer to slow cooker stoneware.
3. In same skillet over medium heat, adding more oil if necessary, cook onion and celery, stirring, until softened, about 5 minutes. Add garlic, oregano and peppercorns and cook, stirring, for 1 minute. Add tomatoes, with juice, and balsamic mixture and bring to a boil. Boil for 1 minute to reduce liquid. Transfer to stoneware and stir thoroughly.
4. Cover and cook on Low for 7 to 8 hours or on High for 4 hours, until eggplant is tender. Stir in olives and capers. Transfer to a serving bowl. Cover and refrigerate for 2 hours or overnight. Garnish with pine nuts and basil, if using.