Place the Italian sausage, sliced bell peppers, sliced onion, minced garlic, crushed tomatoes, Italian seasoning, salt, and black pepper into the slow cooker. Stir to break up the sausage and combine everything evenly.
Cover and cook on low for 7 to 8 hours until the sausage is fully cooked and the peppers are soft.
About 30 minutes before serving, bring the chicken broth to a boil in a saucepan over medium-high heat.
Slowly whisk in the polenta, reduce the heat to low, and cook while stirring often for 20 to 25 minutes until the polenta thickens and becomes creamy.
Spoon the polenta into bowls, top with the sausage and pepper ragu, and finish with grated Parmesan and chopped parsley.