Bring a large pot of water to a rolling boil and add 1 teaspoon of kosher salt. Cook the penne according to package directions until al dente. Drain and set aside.

While the pasta cooks, heat the olive oil in a large skillet over medium heat.

Season both sides of the chicken breasts with 1 tablespoon of the Cajun seasoning. Add the chicken to the skillet and cook for about 3 minutes per side, until the internal temperature reaches 165°F. Remove the chicken from the skillet, wipe the pan clean, and return it to medium heat.

Melt the butter in the skillet, then add the minced garlic and sauté for about 30 seconds until fragrant. Whisk in the flour and cook for another 30 seconds.

Slowly pour in the half-and-half, starting with about 1 tablespoon at a time, whisking constantly. Once you've added about 1/2 cup, you can add the rest more quickly. The sauce should thicken in less than a minute.

Stir in the Parmesan cheese until fully melted. Add the remaining 1/2 tablespoon Cajun seasoning and season with salt and pepper to taste.

Add the cooked penne to the sauce and toss to coat. Top with diced tomato, fresh parsley, extra Parmesan, and the sliced cooked chicken. Serve immediately.