Place the chicken breasts, mushrooms, and onions in the slow cooker.

Combine the heavy cream, sour cream, mustard, Worcestershire sauce, garlic, salt, and pepper in a large bowl.

Pour the cream mixture over the chicken, mushrooms, and onion in the slow cooker. Gently stir everything together.

Cover the slow cooker and cook on low for 7-8 hours or high for 4-5 hours.

Remove the cooked chicken from the slow cooker and shred it. When the chicken is removed, whisk the sauce in the slow cooker so that it is smooth and creamy. Return the shredded chicken to the slow cooker, mixing it in with the sauce.

Just before serving the stroganoff, boil the egg noodles according to the package directions. Drain the cooked noodles, melt the butter, and toss the noodles with the butter until coated.

To serve, plate the noodles and then spoon the chicken, mushrooms, and sauce over the top. Garnish with fresh parsley if desired.