Put the basil, pine nuts, cheese, garlic and salt in a food processor and pulse until finely chopped. Slowly add 1/3 cup olive oil while the machine is running. Scrape down the bowl and add 1 or 2 tablespoons more oil if needed, until pesto is creamy and blended.
Roast the vegetables
Place a large rimmed baking sheet on middle oven rack and heat to 425 degrees. When the oven has reached temperature, remove the hot baking sheet and pour ¼ cup olive oil over the bottom. Arrange the tomatoes and eggplant on the sheet; sprinkle with salt to taste and roast 20 minutes, until browned.
To assemble sandwich
Slice the ciabatta in half horizontally and tear out some of the excess dough; divide the pesto onto both halves of the ciabatta and spread evenly over the bread. Layer the bottom half with the bell peppers, tomatoes, eggplant and mozzarella; grind black pepper over to taste. Close the sandwich and press down firmly.
Wrap sandwich tightly in plastic wrap and place in the refrigerator. Put a brick (or a cast iron skillet weighed down with a few cans) on top of the sandwich to flatten for about 2 hours or overnight.
Unwrap the sandwich and slice into serving pieces.
Customize the filling of your sandwich: Try thinly sliced prosciutto, red onion, roasted zucchini or use goat cheese instead of mozzarella.
Use your favorite prepared pesto sauce instead - you'll need about 1/2 cup.