Steps

1. Preheat a gas grill or build a charcoal fire for direct grilling.
2. Cut the eggplants and zucchini in half lengthwise, then cut the halves lengthwise into slices ½-inch thick. Brush with the some of the oil and sprinkle with kosher salt. Place the slices on the cooking grate over direct medium heat and grill until tender and well marked, 3 to 4 minutes per side. Set aside to cool.
3. Meanwhile, heat enough olive oil to coat the bottom of a large, deep, heavy pot like a Dutch oven or sauté pan over medium heat, then add the onions and celery and cook, stirring occasionally, until golden brown, about 10 minutes.
4. Add the tomatoes, capers, 3 tablespoons of the vinegar, the red pepper, sea salt, and olives, and simmer for 15 minutes. While the tomato mixture is cooking, dice the grilled eggplant and zucchini, then add to the tomatoes and simmer, uncovered, until slightly thickened, another 10 to 15 minutes.
5. Mix the remaining ¼ cup oil and 2 tablespoons vinegar together and stir into the vegetable mixture. Taste and adjust seasonings, adding more vinegar and salt if necessary. Remove from the heat and stir in the basil.
6. Serve warm or cold. This will keep, tightly covered, in the refrigerator for up to 5 days.