01 - Place chicken breasts in the bottom of the slow cooker. Season evenly with salt, pepper, and dried Italian herbs.
02 - Scatter chopped onion and minced garlic over the seasoned chicken.
03 - Pour chicken broth and heavy cream into the slow cooker. Distribute cream cheese cubes and butter pieces across the top.
04 - Cover and cook on low setting for 4 hours, or until chicken reaches internal temperature of 165°F and is tender enough to shred easily.
05 - Remove cooked chicken breasts from slow cooker. Shred using two forks, then return the shredded meat back into the sauce.
06 - Stir uncooked fettuccine into the slow cooker, pressing pasta down to submerge it as much as possible in the liquid.
07 - Sprinkle Parmesan cheese over the mixture and stir thoroughly to combine.
08 - Cover and cook on high heat for 25 to 35 minutes, stirring once or twice during cooking, until pasta is al dente and sauce has thickened to creamy consistency.
09 - Portion hot into bowls and garnish with chopped fresh parsley.