Pat the chicken dry and season with salt, black pepper, Italian seasoning, and paprika.
Place the chicken in the slow cooker. Add the olive oil, minced garlic, chicken broth, sun-dried tomatoes, onion powder, and optional red pepper flakes.
Cover and cook on LOW for approximately 3–4 hours for chicken breasts or 4–5 hours for thighs, checking the internal temperature and removing the chicken when it reaches 165°F (74°C).
Remove the cooked chicken temporarily. Stir the heavy cream and grated Parmesan into the cooking liquid.
Return the chicken to the slow cooker. Cover and cook on LOW for another 15–25 minutes, until the sauce is heated through. Avoid boiling.
For a thicker sauce, mix the cornstarch and cold water until smooth. Stir into the sauce and cook briefly until thickened.
Taste the sauce and adjust the seasonings if needed. Garnish with chopped parsley and serve with pasta, rice, mashed potatoes, or vegetables.