Cook over medium-high heat for 3 minutes. Reduce heat and simmer 15 minutes. Heat oven to 350 degrees. Add zucchini, green pepper and black pepper to eggplant mixture. Simmer for only 10 minutes more.
Place rolls on baking sheet and toast in oven.
Leave 4 halves on baking sheet and set aside other 4 halves. Top 4 halves on baking sheet with ratatouille. Sprinkle with Parmesan cheese. Return to oven for 5 minutes. Remove and top with other roll halves.