Step 1: Pat chicken thighs dry with paper towels. Season both sides with 1/2 tsp salt and 1/4 tsp black pepper.
Step 2: In a large skillet, heat olive oil over medium-high heat. Sear chicken thighs for 2-3 minutes per side until lightly browned. This step is optional but adds great flavor. Transfer chicken to a 6-quart (or larger) slow cooker.
Step 3: In the same skillet (or directly in the slow cooker if it has a sear function), add the minced garlic and chopped sun-dried tomatoes. Sauté for 1-2 minutes until fragrant.
Step 4: Deglaze the skillet with a splash of chicken broth if needed, scraping up any browned bits. Pour the remaining chicken broth, heavy cream, Italian seasoning, red pepper flakes (if using), 1/2 tsp salt, and 1/4 tsp black pepper into the slow cooker over the chicken and aromatics. Stir gently to combine.
Step 5: Cover and cook on low for 3-4 hours or on high for 2-3 hours, or until chicken is cooked through and very tender (internal temperature 165°F/74°C).
Step 6: Carefully remove the cooked chicken from the slow cooker and shred it using two forks. Set aside.
Step 7: Stir the grated Parmesan cheese and fresh spinach (if using) into the sauce in the slow cooker. Cook, stirring occasionally, until the spinach has wilted (about 5 minutes).
Step 8: Return the shredded chicken to the slow cooker and stir to coat thoroughly with the creamy sauce.
Step 9: Taste and adjust seasoning as needed. Garnish with fresh basil or parsley before serving. Serve hot over pasta, rice, or mashed potatoes.