Prep the slow cooker. Lightly spray the crock with nonstick spray or use a liner for easy cleanup.
Layer the base. Add the sliced mushrooms and chopped onion to the bottom of the crockpot. Sprinkle in the garlic, thyme, paprika, pepper, and salt.
Place the chicken on top. Arrange the breasts or thighs in a single layer over the vegetables.
Mix the sauce. In a bowl, whisk together the cream of mushroom soup and chicken broth until smooth. Pour this mixture evenly over the chicken.
Set and cook. Cover and cook on Low for 4–6 hours or High for 2.5–3.5 hours, until the chicken is tender and reaches 165°F internally.
Add the creamy finish. About 20 minutes before serving, stir in the cream cheese (or heavy cream).
If you want a thicker sauce, stir in the cornstarch slurry at the same time. Cover and let it melt and thicken.
Shred or slice. You can shred the chicken right in the crockpot for a saucier, mix-in style, or slice and spoon the sauce over the top.
Taste and finish. Adjust salt and pepper as needed. Garnish with chopped parsley.
Serve. Spoon over rice, buttered egg noodles, or mashed potatoes.
Add a side of broccoli or green beans for balance.