Brown the sausage links in a skillet over medium heat until lightly colored on all sides. Place them in the slow cooker.
Add the sliced bell peppers, sliced onion, and minced garlic to the slow cooker.
Pour in the crushed tomatoes. Sprinkle with oregano, salt, and black pepper. Stir gently to combine.
Cover and cook on low for 8 hours.
About 30 minutes before serving, bring the chicken broth to a boil in a saucepan. Whisk in the cornmeal gradually.
Reduce heat to low and cook the polenta, stirring often, until thick and creamy, about 20 minutes.
Stir the butter into the polenta, then remove from heat. Slice the cooked sausage and return it to the ragù.
Spoon the polenta into bowls and top with the ragù. Sprinkle with Parmesan cheese before serving.