Boil a salted pot of water for the pasta and cook it al dente according to package directions.
Meanwhile, cut the chicken in half lengthwise so you've got 4 thinner cutlets. Season the chicken with the first 1/2 tablespoon of Cajun seasoning, the garlic powder, and salt & pepper (go easy on the salt if your Cajun seasoning contains some salt), then coat each piece in flour and shake off any excess.
To a skillet over medium-high heat, add the olive oil and 1 tablespoon of the butter. Let the pan warm up for a few minutes.
Cook the chicken for 5 minutes/side or until golden and cooked through. Transfer the chicken to a plate and set it aside.
Reduce the heat to medium, and add the remaining 1 tablespoon butter to the skillet and once it's melted, add the garlic and 1 teaspoon of flour. Let it cook for about a minute, stirring nearly constantly.
Whisk in the cream, lemon juice, Dijon mustard (if using), and the remaining 1/2 tablespoon of Cajun seasoning.
Stir in the tomato and let the sauce thicken up a bit (it shouldn't take longer than a few minutes). Add a splash of the hot pasta water if it gets too thick.
Drain the pasta and add it to the skillet and toss. Stir in the basil/parsley/scallions if using. Taste and season with extra salt & pepper as needed (be generous if your Cajun seasoning doesn't contain any salt). Serve with the chicken sliced on top (or leave each piece whole) and freshly grated parmesan.