Season chicken breasts with salt, pepper, and garlic powder. Heat olive oil in a skillet over high heat and sear chicken breasts on both sides until browned.
Transfer seared chicken breasts to the slow cooker. Add sliced sausage, heavy cream, chicken broth, butter, smashed garlic cloves, 1/2 teaspoon kosher salt, Cajun seasoning, and freshly ground black pepper.
Cook on low for 4 hours or high for 2-3 hours, until chicken reaches an internal temperature of 165°F (74°C).
Remove chicken from slow cooker and let cool, then slice.
Stir in hot water and uncooked penne pasta. Cover and cook on high for about 30 minutes, until pasta is tender.
Once pasta is cooked, stir in Parmesan cheese, sliced chicken, and additional salt and black pepper to taste.
