Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package instructions until al dente, usually about 10-12 minutes. Drain the pasta and set aside, reserving about ½ cup of pasta water for the sauce.
While the pasta cooks, heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Toss the diced chicken breast with 1 tablespoon of Cajun seasoning, salt, and pepper. Add the chicken to the skillet and cook for 5-7 minutes, stirring occasionally, until the chicken is cooked through and slightly browned. Remove the chicken from the skillet and set aside.
In the same skillet, add the chopped onion and bell pepper. Sauté for about 4-5 minutes until softened and fragrant. Add the minced garlic and cook for an additional 1 minute, making sure not to burn the garlic.
Add the drained diced tomatoes to the skillet and stir to combine. Let the mixture simmer for 2-3 minutes to meld the flavors. Slowly pour in the heavy cream while stirring. Add salt and pepper to taste. If the sauce looks too thick, add a splash of the reserved pasta water to loosen it up.
Return the cooked chicken to the skillet and toss to coat in the creamy Cajun sauce. Add the drained penne pasta and gently mix everything together until the pasta is well coated and heated through.
Remove the skillet from heat. Sprinkle freshly chopped parsley over the top for a bright, fresh finish. Serve immediately with your favorite side or a simple green salad.