Pat the chicken dry and season both sides with a pinch of salt and pepper. This step helps develop a bit of color and flavor even though the chicken will finish in the slow cooker.
Heat the olive oil in a skillet over medium-high heat. Sear the chicken for about 2-3 minutes per side, just until lightly browned. You’re not cooking it through here; you’re building flavor on the surface.
Transfer the seared chicken to the slow cooker. In the same skillet, add the onion and a pinch of salt; sauté for 2-3 minutes until translucent. Add the garlic and cook for 30 seconds until fragrant.
Pour in the chicken stock and scrape up any browned bits from the pan. Stir in the sun-dried tomatoes and Italian seasoning, then pour this mixture over the chicken in the slow cooker.
Add the heavy cream and grated Parmesan to the slow cooker. Give everything a gentle stir so the sauce begins to come together around the chicken.
Cover and cook on low for 6-7 hours, or until the chicken is tender and cooked through. About 20 minutes before serving, stir in the spinach until wilted and the sauce is nicely creamy.
Sprinkle the mozzarella over the top, close the lid, and let it melt for a few minutes. If you like a little extra body to the sauce, you can remove the lid briefly to thicken it slightly.
Taste and adjust the seasoning with a bit more salt or pepper if needed. Serve spoonfuls of the creamy sauce over the chicken, with or without pasta, rice, or crusty bread to soak up every last bit.