Build a charcoal fire or preheat a gas grill. Soak three 12-inch bamboo skewers in hot water for 30 minutes.
Thread the onion slices onto the skewers like lollipops, brush with olive oil and sprinkle with salt. Set aside. In a medium bowl, drizzle the eggplant and zucchini strips with olive oil and sprinkle with salt. Toss to coat.
Place the zucchini, eggplant and onion on the grill, cover, and grill over direct medium heat until tender, 3 to 4 minutes per side. Transfer to a platter to cool, then coarsely chop.
In a deep, medium skillet, heat 2 tablespoons olive oil over direct medium-high heat. Add the celery, season with salt and cook until golden brown, about 8 minutes. Add the tomatoes, olives, vinegar, capers and crushed red pepper, if using, and simmer for 15 minutes.
Add the zucchini, eggplant and onion to the tomato mixture and simmer until slightly thickened, 10 to 15 minutes. Remove from the heat, stir in the basil and season to taste with salt; set aside.
Brush the cut sides of the baguette with olive oil and grill, covered, over indirect, medium-low heat until marks appear, 3 to 6 minutes. Transfer to a work surface and cut into 1-inch-thick slices. Serve alongside the caponata.