Boil the pasta. Fill a large pot with water, add a generous pinch of salt, and bring to a boil. Add the penne and cook according to package directions until al dente. Reserve 1/2 cup pasta water, then drain the pasta.
Season the chicken. In a bowl, toss chicken pieces with 1 tablespoon Cajun seasoning, garlic powder, onion powder, smoked paprika, salt, and black pepper until evenly coated.
Sear the chicken. Heat 1 tablespoon olive oil in a skillet over medium-high heat. Cook chicken until golden browned and nearly cooked through, about 5 minutes per side.
Cook the vegetables. In the same skillet, add remaining 1 tablespoon oil. Sauté diced onions and bell peppers until tender, 3-4 minutes. Add minced garlic and cook for 1 minute.
Create the cream sauce. Combine chicken broth, heavy cream, Parmesan, tomato paste, and remaining 1 tablespoon Cajun seasoning into the skillet with the vegetables and chicken. Stir in butter until melted. Simmer for 5-7 minutes until thickened, adding pasta water as needed. Toss with pasta and garnish with parsley.