Season the chicken: Pat the chicken dry. Sprinkle with salt, pepper, Italian seasoning, garlic powder, and onion powder on both sides.
Layer the aromatics: Add the minced garlic and diced onion (if using) to the bottom of the crockpot. Pour in the chicken broth.
Add the chicken: Place the seasoned chicken on top of the aromatics in an even layer.
Slow cook: Cover and cook on Low for 4–5 hours or on High for 2–3 hours, until the chicken is tender and reaches 165°F internally.
Shred the chicken: Remove the chicken to a cutting board.
Shred with two forks into bite-size pieces. Leave the cooking liquid in the crockpot.
Make it creamy: Add heavy cream, cream cheese, butter, and Parmesan to the crockpot. Whisk or stir until the cream cheese softens and the mixture is mostly smooth.
Finish the sauce: Return the shredded chicken to the crockpot.
Stir to coat. Cover and cook on Low for another 15–20 minutes to thicken and meld flavors.
Adjust and brighten: Stir in lemon juice (if using). Taste and add more salt or pepper as needed.
If using spinach, fold it in and let it wilt for 3–4 minutes.
Serve: Spoon the creamy chicken Alfredo over warm zucchini noodles, spaghetti squash, or steamed broccoli. Top with extra Parmesan and parsley.