Spray 4- to 5-quart slow cooker with cooking spray. In 12-inch skillet, heat 2 tablespoons of the oil over medium-high heat. Cook eggplant and onion in oil 5 minutes, stirring occasionally, until almost tender. Spoon into slow cooker.
In same skillet, heat 1 tablespoon oil over medium-high heat. Cook zucchini and bell peppers in oil 3 minutes, stirring occasionally. Spoon into slow cooker. Add remaining ingredients except polenta and remaining oil; stir well.
Cover; cook on Low heat setting 4 to 6 hours.
In 12-inch nonstick skillet, heat remaining 2 tablespoons oil over medium-high heat. Cook polenta slices in oil 4 to 6 minutes on each side until golden brown. Place polenta on serving plates; top with stew.