Heat oil in a medium saucepan over medium heat. Add bell peppers, chopped onion, and garlic cloves, and sauté for 6 minutes. Stir in salsa, cumin, black beans, and tomatoes, and bring to a boil. Cover, reduce heat, and simmer for 10 minutes, stirring occasionally.
Quarter each polenta slice. Place polenta in a shallow 2-quart microwave-safe baking dish. Microwave, uncovered, at high for 2 minutes or until hot, tossing gently after 1 minute.
Divide the polenta evenly among 4 bowls, and spoon 3/4 cup of bean stew over the polenta. Top each serving with 1 tablespoon sour cream, 1 tablespoon cilantro, and 2 1/2 tablespoons avocado.