Cook spaghetti according to package instructions until al dente. Drain and set aside.
- Slice the chicken breasts into thin strips and season generously with Cajun seasoning.
- Heat olive oil in a large skillet over medium heat. Add the seasoned chicken and cook until browned and fully cooked, about 5–7 minutes.
- Remove the chicken from the skillet and set aside.
- In the same skillet, melt butter and sauté garlic (and onion, if using) until fragrant.
- Add heavy cream and bring to a gentle simmer. Stir in Velveeta cheese and mozzarella until fully melted and smooth.
- Season the sauce with salt and black pepper to taste.
- Return the cooked chicken to the skillet and stir to coat with the sauce.
- Add the cooked spaghetti and toss until well combined and evenly coated.
- Garnish with chopped parsley and serve hot.