1. Freshly grated Pecorino Romano or Parmesan cheese, for serving (optional).
2. Season the chicken with the salt and pepper to taste. Heat a large nonstick skillet over medium-high heat. Coat with cooking spray, add the chicken, and cook until browned, 2 to 3 minutes per side. Transfer to a slow cooker.
3. Reduce the heat under the skillet to medium and coat with more cooking spray. Add the garlic and onion and cook, stirring, until soft, 3 to 4 minutes. Transfer to the slow cooker and add the tomatoes, bell peppers, mushrooms, thyme, oregano, and bay leaf. Stir to combine.
4. Cover and cook on high for 4 hours or on low for 8 hours.
5. Discard the bay leaf and transfer the chicken to a large plate. Pull the chicken meat from the bones (discard the bones), shred the meat, and return it to the sauce. Stir in the parsley. If desired, serve topped with Romano or Parmesan.