Preheat oven to 400F. Place the eggplants on a baking sheet and prick with a fork. Bake until tender but not mushy, about 35-45 minutes.
When the eggplants are cool enough to handle, remove the peel and dice the flesh into 1-inch cubes. Place the cubes in a strainer or drainer. In a large saute pan over moderate heat, bring the stock to a boil.
Add the onions and cook until tender, about 10 minutes. Add the celery and cook for 5 minutes. Add the tomato puree, vinegar, and sugar, and simmer a few minutes.
Stir in ther eserved eggplant, capers, and raisins and warm through. Adjust the seasoning with salt and pepper. Serve at room temperature.
This can be made a few days ahead of time. Bring to room temperature for serving, check seasoning, and add vinegar and salt if desired.