Heat the olive oil in a large skillet over medium heat.
Add the diced chicken along with the salt, black pepper, and Cajun seasoning.
Cook until the chicken is golden brown and fully cooked through, stirring occasionally.
Remove the chicken from the skillet and set aside.
Bring a large pot of salted water to a boil.
Cook the penne pasta according to package directions, about 10 minutes, until al dente.
While the pasta cooks, return the skillet to medium-low heat.
Add the butter and allow it to melt.
Stir in the minced garlic and cook for about 1 minute, until fragrant.
Add the flour, salt, black pepper, and Cajun seasoning. Whisk continuously until a thick golden paste forms.
Slowly pour in the heavy cream while whisking constantly to prevent lumps from forming.
Allow the sauce to cook for about 3-4 minutes over a low simmer, whisking frequently. Do not allow the sauce to boil.
Stir in the freshly grated Parmesan cheese and remove the skillet from the heat.
Mix until the cheese is completely melted and the sauce is smooth and creamy.
Add the cooked chicken and drained pasta to the Alfredo sauce.
Toss until everything is evenly coated.
Top with diced tomatoes and fresh parsley before serving.