Heat butter in sauté pan over medium-high heat. Sauté onions and bell peppers for 1 to 2 minutes. Add garlic, eggplant, zucchini, salt and pepper. Cook 3 to 4 minutes, until vegetables are crisp-tender.
Add tomatoes; cook until mixture has thickened. Set aside.
To assemble sandwiches, layer bottom half of hoagie bun with 3 ounces roast beef and 2 slices Italian Blend cheese. Place under broiler until cheese is melted. Top with 3 ounces ratatouille and top half of bun.