Add the diced onion and minced garlic to the bottom of the slow cooker. Arrange the chicken on top.
In a small bowl or measuring cup, whisk together the chicken broth, Worcestershire sauce, parsley, garlic powder, onion powder, paprika, salt, and black pepper.
Pour the seasoned broth mixture over the chicken. Place the butter on top.
Cover and cook on low for 5–6 hours or high for 3–4 hours, until the chicken is tender and reaches an internal temperature of 165°F.
Transfer the cooked chicken to a plate and shred it using two forks.
Whisk together the corn starch and cold water until completely smooth. Stir the slurry into the cooking liquid in the slow cooker.
Cover and cook on high for 10–15 minutes, or until the gravy begins to thicken.
Stir in the heavy cream or whole milk. Return the shredded chicken to the slow cooker and stir until coated in the gravy.
Cook for another 5–10 minutes, until everything is warmed through. Taste and add additional salt or pepper if needed. Serve warm over mashed potatoes, rice, noodles, or biscuits.