Bring water to boil in a large pot, add 1 tbsp of salt, and cook penne pasta according to instructions on the packaging until it's al dente.
Drain the pasta and set 1/2 cup of pasta water aside.
Using a meat tenderizer, beat the chicken breasts until they are around 1/2 inch in thickness. Cover the chicken breasts with 1 tbsp Cajun seasoning on both sides, then salt and black pepper.
Heat up a large, non-stick skillet over medium-high heat. Add in olive oil and sear chicken on both sides. About 4 minutes on each side. Then, lower the heat and sauté until the chicken cooks through. It should be another 4 minutes per side.
Once cooked, remove from the pan onto the cutting board. Slice into thin strips and cover with a plate to keep them warm.
In the same skillet with any of the leftover fat in it, add 1 tbsp of butter. Then, press in garlic cloves and cook for 30s.
Add the diced tomatoes and let it cook for another 2 minutes or so. Finally, pour in the heavy cream and add the remaining 1 tbsp of Cajun seasoning and parmesan cheese. Give it a stir and bring to a simmer.
Add in the cooked penne pasta, then the cooked chicken, and pour in the 1/2 cup of pasta water. Toss until combined and let the ingredients warm up. Add more salt and black pepper if needed.
Garnish with grated parmesan and parsley before serving.