Brown the Italian sausage in a skillet over medium heat, breaking it into small pieces as it cooks. Drain excess fat and transfer the sausage to the slow cooker.
Add the cubed butternut squash, diced onion, minced garlic, crushed tomatoes, chicken broth, oregano, basil, salt, and black pepper to the slow cooker. Stir to combine.
Cover and cook on low for 6 to 7 hours until the squash is very tender and the sauce has thickened.
Near the end of the cooking time, bring the water to a boil in a medium pot. Whisk in the cornmeal gradually. Reduce heat to low and cook, stirring often, for 15 to 20 minutes until the polenta is thick and creamy. Stir in the butter and Parmesan cheese.
Spoon the polenta into bowls and top with the sausage ragout. Sprinkle with fresh parsley before serving.