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 until golden brown on both sides, about 2-3 minutes per side. Remove from heat.
Place the seared chicken breasts into the slow cooker.
In a medium bowl, whisk together the minced garlic, chicken broth, heavy cream, grated Parmesan cheese, Italian seasoning, and crushed red pepper flakes until well combined.
Pour the creamy garlic Parmesan mixture over the chicken breasts in the slow cooker.
Add the unsalted butter on top of the chicken and sauce.
Cover and cook on low for 4 hours, or until the chicken is cooked through and tender.
Once cooked, sprinkle chopped fresh parsley over the chicken and sauce before serving.
Serve the chicken with the creamy garlic Parmesan sauce spooned over, ideally with pasta, rice, or steamed vegetables.