Prep the base: Add sliced onions and mushrooms to the bottom of your crockpot. Scatter the garlic on top.
Season the chicken: Sprinkle the chicken with salt, pepper, and paprika.
Lay the pieces over the vegetables.
Mix the liquids: In a bowl, whisk together the chicken broth, Worcestershire sauce, and Dijon mustard. Pour over the chicken. Dot with butter if using.
Slow cook: Cover and cook on Low for 4–5 hours or on High for 2–3 hours, until the chicken is very tender and easily shreds.
Shred the chicken: Transfer the chicken to a cutting board and shred with two forks, or cut into bite-size pieces.
Return it to the crockpot.
Add the creaminess: Stir in the cream cheese until mostly melted and smooth. Add the sour cream and mix well. If you prefer a thicker sauce, stir in the cornstarch slurry and cook on High for 10–15 minutes, stirring once or twice, until it reaches your desired consistency.
Taste and adjust: Add more salt and pepper if needed.
If the sauce tastes flat, a small splash of Worcestershire or a squeeze of lemon brightens it up.
Serve: Spoon the stroganoff over warm egg noodles, rice, or mashed potatoes. Garnish with fresh parsley.