Season the chicken: Pat the chicken dry. Sprinkle with salt, pepper, garlic powder, onion powder, and Italian seasoning on both sides.
Build the base: Add the chopped onion and garlic to the slow cooker.
Pour in the chicken broth. Stir in the cream of chicken soup and Dijon mustard until mostly smooth.
Add the chicken: Nestle the seasoned chicken into the sauce mixture. If using mushrooms, add them now.
Hold off on peas and carrots until the end so they don’t get mushy.
Cook low and slow: 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 in the center.
Add cream cheese: About 30 minutes before serving, add the cream cheese cubes. Cover and let them melt, then stir until the sauce is smooth and creamy.
Shred or slice: Remove the chicken to a cutting board and shred with two forks or slice it. Return to the slow cooker and stir to coat in the sauce.
Finish with veggies: If using frozen peas and carrots, stir them in now.
Cover and cook on Low for 10–15 minutes until heated through.
Taste and adjust: Add more salt or pepper if needed. The sauce should be savory, creamy, and well-seasoned.
Serve: Spoon the creamy chicken over rice, egg noodles, or mashed potatoes. Sprinkle with fresh parsley for a bright finish.