Add the Italian sausage links, sliced red and green bell peppers, sliced onion, and minced garlic to the slow cooker.
Pour the crushed tomatoes over the mixture and sprinkle in the Italian seasoning. Stir gently to combine everything.
Cover and cook on low for 6-8 hours until the sausages are fully cooked and the vegetables are soft.
About 30 minutes before the ragu finishes, bring the chicken broth to a boil in a saucepan. Whisk in the polenta, reduce the heat to low, and cook while stirring often until the polenta thickens, about 20-25 minutes.
Stir the butter and half the Parmesan cheese into the polenta. Slice the cooked sausages and serve the ragu over the polenta. Top with the remaining Parmesan and fresh basil.