1. In 12-inch skillet over medium heat, in hot olive or salad oil, cook onion, stirring occasionally, until tender-crisp. Add eggplant and green peppers; cook 10 minutes, stirring frequently. Remove skillet from heat.
2. Preheat oven to 350°F. In 2 1/2-quart casserole, combine vegetable mixture, zucchini, tomatoes with their liquid, salt, and sugar. Cover casserole and bake 35 minutes or until vegetables are tender.
3. Meanwhile, in same skillet over medium heat, heat sausages and water to boiling. Cover; simmer 5 minutes. Remove cover; continue cooking, turning sausages frequently, until sausages are well browned, about 20 minutes. Remove sausages to paper towels to drain. Slice sausages into 1/2-inch pieces.
4. Remove casserole from oven; stir sausages into vegetable mixture; sprinkle with Swiss cheese. Bake casserole, uncovered, 10 minutes or until heated through.