This is like a Niçoise salad in sandwich form. Our take is a little unconventional with the addition of roasted eggplant, but you can leave it out, if desired, or substitute cooked artichoke hearts.
Preheat the oven to 400°F. Slice lengthwise into ¼-inch-thick slabs:
Flatten with the palm of your hand:
Brush both sides of the eggplant slices and bell pepper with a little olive oil. Place them on a baking sheet and roast until tender, 15 to 20 minutes. Cool the peppers and eggplant slightly. When cool enough to handle, remove the skin from the pepper.
Meanwhile, combine in a medium bowl:
Split horizontally:
If using a round loaf, remove some of the soft inner bread, creating a cavity. Lightly toast the bread, then rub over the toasted surfaces:
Cover the bottom piece(s) of bread with:
Pile the tuna mixture on top of the basil. Top with the roasted eggplant, bell pepper, and:
Cover with the top piece(s) of bread, and wrap tightly in several layers of plastic wrap. Place on a rimmed baking sheet or cutting board and weight down with a cast-iron skillet or other heavy weight (or, if you’d like to follow M.F.K. Fisher’s advice, have someone sit on it). Allow the sandwiches to marinate, refrigerated, for at least 1 hour and up to 24 hours.