Place the chicken breasts on a cutting board, pat them dry, and rub them with the Cajun seasoning on both sides. Melt two tablespoons of butter in a large skillet and brown the chicken for 3 minutes on each side. Transfer to a plate and keep warm.
Cook the pasta according to package instructions, drain, and set aside. Using the same skillet, melt the other two tablespoons of butter, and saute the minced garlic and onion for about a minute.
Pour in the half and half, season with salt and pepper, and simmer (not boil) for about 3-4 minutes. Add the Parmesan cheese and remove from heat. Season with salt and pepper.
Add the cooked and drained pasta to the skillet and toss to coat. Cook for about a minute or until pasta is hot again. Serve in individual bowls and top with the sliced chicken, chopped fresh tomato, and chopped parsley.