Place the Italian sausages, sliced bell peppers, sliced onions, minced garlic, dried oregano, salt, black pepper, and chicken broth into the slow cooker.
Cover and cook on low for 6 to 7 hours until the sausages are cooked through and the vegetables are soft.
About 20 minutes before serving, bring the water to a boil in a medium saucepan. Slowly whisk in the polenta.
Reduce the heat to low and cook the polenta for 15 minutes, stirring often, until it thickens.
Stir the butter and grated Parmesan cheese into the polenta until fully combined.
Slice the cooked sausages and spoon them with the peppers and onions over servings of polenta.