Place the chicken breasts in the bottom of the slow cooker.
Add the sliced mushrooms, bell peppers, and onion on top of the chicken.
Sprinkle the minced garlic over the vegetables.
Pour the crushed tomatoes and chicken broth over everything.
Add the Italian seasoning, salt, and black pepper.
Stir gently to combine the sauce ingredients without moving the chicken.
Cover and cook on low for 6 hours or until the chicken reaches an internal temperature of 165°F.
Remove the chicken, shred or slice it, and return it to the sauce.
Stir in the fresh parsley just before serving.