Heat 1/4 cup of the oil in a heavy large skillet over medium heat. Add the celery
and saute until crisp-tender, about 2 minutes. Add the eggplant and saute until beginning to soften, about 2 minutes. Season with salt. Add the red pepper
and cook until crisp-tender, about 5 minutes. Add the onion
and saute until translucent, about 3 minutes. Add the diced tomatoes
with their juices, raisins, and oregano. Season with salt and pepper, to taste. Simmer
over medium-low heat until the flavors blend
and the mixture thickens, stirring often, about 20 minutes. Add the vinegar, sugar, and capers
. Season with salt and pepper, to taste.
Cut the bread pieces horizontally in half. Grill the bread cut-side down on a grill pan
over medium-high heat until golden, about 2 minutes. Rub the whole garlic cloves over the toasted side of the bread. Add a slice of mozzarella
to bread bottom, spoon the caponata
on top, cover with the other bread half, and serve.
To take for a picnic: Wrap the sandwiches separately in plastic wrap
or parchment paper. Chill until ready to serve, or keep at room temperature up to 1 hour.