Add the sliced onion, carrots, celery, mushrooms (if using), and minced garlic to the bottom of a 4–5 quart slow cooker.
Place the chicken thighs on top of the vegetables. Sprinkle with salt, pepper, paprika, dried parsley, and the pot roast seasoning mix.
Pour in the chicken broth and Worcestershire sauce. Top with butter slices, rosemary sprig, and bay leaf.
Cover and cook on LOW for 5–6 hours (or on HIGH for about 3½ hours), until the chicken is tender and easily shredded. Once done, remove the chicken from the pot and shred or cut into large chunks. Discard the rosemary and bay leaf.
To make a thicker gravy, mix 1 tablespoon cornstarch with 1 tablespoon warm broth in a small bowl. Whisk until smooth, then stir it back into the slow cooker. Cover and cook on HIGH for 15–20 minutes, or until slightly thickened.
Place a scoop of mashed potatoes on each plate. Top with the tender chicken and vegetables, then pour the warm gravy over everything. Garnish with a sprinkle of parsley if desired.
