Bring a large pot of salted water to a rolling boil. 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. Place them in the skillet and cook for about 3 minutes per side, until the internal temperature reaches 165°F. Remove the chicken and set aside.

Wipe the skillet clean, then return it to medium heat. Melt the butter and sauté the garlic for about 30 seconds. 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 about 1/2 cup has been added, 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, plus salt and pepper to taste.

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