- Preheat the grill, setting the burners to high. Oil the grate.
- Grill the bell peppers for about 5 minutes on each side or until they are tender and charred. Place in an airtight container, close the lid and let cool for 5 minutes. Peel the skins.
- Meanwhile, grill the zucchini for 2 to 3 minutes and the eggplant for 1 to 2 minutes on each side or until tender. Place the vegetables in a large dish. Let cool. Add the peeled peppers.
- Drizzle the vegetables with the oil and vinegar. Season with salt and pepper, and toss well. Set aside.
- On a work surface, cut off the top of the bread loaf with a bread knife. Hollow out the loaf by removing about 4 cups of the crumb, leaving about a ½-inch (1.5 cm) thick crust. Keep the crumb for another use. Set aside.
- Spread the ajvar inside the loaf and on the cut side of the bread top. Evenly layer the mortadella, calabrese, vegetables and basil inside the bread. Close the loaf with the bread top. Cover with aluminum foil or plastic wrap and place on a plate. Weigh down with cans or plates directly placed on the loaf and refrigerate for 8 hours or overnight.
- Leave the muffuletta at room temperature for at least 1 hour before serving.
Ajvar is a Balkan spread made from puréed red bell peppers, eggplant and garlic. It can be found in certain grocery stores. If unavailable, use homemade or store-bought roasted red pepper purée.