Preheat oven to 450°.
Place eggplant in a single layer on a baking sheet coated with cooking spray. Lightly coat eggplant with cooking spray; sprinkle with 1/4 teaspoon black pepper and 1/8 teaspoon salt. Place bell pepper rings and onion slices on another baking sheet coated with cooking spray. Drizzle with oil, and sprinkle with remaining 1/4 teaspoon black pepper and 1/8 teaspoon salt.
Bake eggplant at 450° for 12 minutes; turn slices over. Add bell pepper and onion to oven with eggplant; bake at 450° for 10 minutes; turn bell pepper and onion slices over. Bake an additional 10 minutes or until vegetables are tender and well browned. Remove pans from oven. Drizzle balsamic vinegar over bell pepper and onion; toss to coat.
Cut bread in half horizontally. Place halves, cut sides up, on a baking sheet. Bake at 450° for 3 minutes or until bread is lightly toasted. Remove top half of loaf from pan; set aside.
Spread 1 tablespoon pesto on each half of bread. Arrange roasted eggplant slices on bottom half of bread; top with bell pepper and onion mixture, and sprinkle with cheese. Broil 1 minute or until cheese melts; replace top half of loaf. Cut sandwich into 4 pieces, and serve immediately.
Note: To roast the vegetables in one oven, use both racks. Start the eggplant on the top rack. When the eggplant slices are turned, move them to the bottom rack, and place the bell pepper and onion on the top rack.