Combine the stock, tomatoes (with their liquid), beans, carrots, celery, onion, thyme, sage, bay leaves, and pepper in the slow cooker.
3 cups vegetable or chicken stock, 28 ounces whole peeled tomatoes, 15 ounces white kidney beans, 2 carrots, 1 stalk celery, 1 cup onion, 1 teaspoon dried thyme, ½ teaspoon dried sage, 2 bay leaves, ½ teaspoon freshly ground pepper