Rinse eggplant, squash, bell peppers, and rosemary sprigs. Scrub potatoes. Trim and discard ends of eggplant and squash. Stem and seed bell peppers. Cut eggplant, squash, and bell peppers into 1 1/2-inch chunks and potatoes into halves. Combine vegetables, rosemary, olive oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper in an 11- by 17-inch roasting pan (2 to 2 1/2 in. deep); mix well.
Bake in a 400º regular or convection oven, stirring occasionally, until potatoes are browned and tender when pierced, about 1 hour. Mound on a large platter. Add more salt and pepper to taste. Serve warm or at room temperature.