To make in a slow cooker, heat cooking spray in a skillet. Add onion, celery, and carrot. Cook, stirring often, 3 minutes, and then spoon into slow cooker. Stir garlic, tomato paste, sweet potato, corn, zucchini, and broth into slow cooker.
Cover and cook for 6 hours on low or 2 1/2 hours on high. Remove lid and stir in lemon juice. Serve garnished with basil. Serving size: 1/6th of soup