Season the chicken breasts evenly with salt and black pepper on both sides.
Heat olive oil in a large skillet over medium-high heat. Add the chicken breasts and sear each side for 2-3 minutes until golden brown. Remove chicken and set aside.
Add minced garlic to the skillet and sauté for 1 minute until fragrant.
Pour in chicken broth, scraping the bottom of the skillet to loosen any browned bits.
Transfer the garlic and broth mixture to the slow cooker.
Place the seared chicken breasts into the slow cooker.
Add heavy cream, grated Parmesan cheese, dried Italian seasoning, chopped sun-dried tomatoes, and chopped spinach to the slow cooker. Stir gently to combine.
Cover and cook on low for 4 hours, or until the chicken is cooked through and tender.
About 15 minutes before serving, mix cornstarch and water in a small bowl to create a slurry.
Stir the slurry into the slow cooker to thicken the sauce. Cover and cook for an additional 15 minutes.
Taste and adjust seasoning with salt and pepper if needed.
Serve the chicken breasts topped with the creamy spinach sauce.