Place the chicken thighs in the bottom of the slow cooker. Add the sliced onion, bell peppers, and mushrooms on top.
Pour the crushed tomatoes over the vegetables. Add the minced garlic, oregano, basil, salt, and pepper.
Cover and cook on low for 6 to 7 hours until the chicken is tender and pulls apart easily.
About 30 minutes before serving, bring the water to a boil in a saucepan. Whisk in the polenta and reduce heat to low.
Cook the polenta for 20 to 25 minutes, stirring often, until thick and creamy. Stir in the butter until melted.
Shred the chicken lightly in the slow cooker and stir the sauce. Spoon the chicken mixture over bowls of polenta and top with Parmesan.