Season the chicken: Pat the chicken dry. Sprinkle both sides with salt, pepper, Italian seasoning, and garlic powder.
Optional sear: For extra flavor, heat butter or olive oil in a skillet and sear the chicken for 2–3 minutes per side until lightly golden.
This step is optional but adds nice color.
Layer the crockpot: Place sliced onion (if using) in the bottom of the slow cooker. Add the chicken on top. Scatter minced garlic and sun-dried tomatoes over the chicken.
Add liquids: Pour in the chicken broth.
Cover and cook on Low for 4–5 hours or High for 2–3 hours, until the chicken is cooked through and tender.
Make it creamy: Stir in the heavy cream, Parmesan, and cream cheese (if using). Add a pinch of red pepper flakes if you like a mild kick.
Add spinach: Stir the spinach into the sauce and cover for 5–10 minutes until wilted. Taste and adjust salt and pepper.
Serve: Garnish with fresh basil or parsley.
Serve over pasta, rice, mashed potatoes, or with crusty bread to soak up the sauce.