Start by bringing a large pot of salted water to a boil. Add the penne or bow tie pasta and cook according to package instructions until al dente. Drain the pasta, reserving about ½ cup of the cooking water, and set it aside.
While the pasta is cooking, season the chicken breasts with Cajun seasoning on both sides. In a large skillet, heat the olive oil over medium-high heat. Once hot, add the chicken breasts and cook for about 6-7 minutes on each side, or until cooked through and golden brown. Remove from skillet and let rest a few minutes before slicing into strips.
In the same skillet, reduce heat to medium and add the butter. Once melted, add the diced onion and cook until translucent, about 3-4 minutes. Add the minced garlic and sauté for an additional minute until fragrant.
Pour in the heavy cream, stirring to combine with the onion and garlic. Bring to a gentle simmer. Stir in the lemon zest and freshly grated Parmesan cheese, mixing until cheese has melted and sauce is smooth. If sauce is too thick, add a splash of reserved pasta water to desired consistency.
Add the cooked pasta and sliced chicken to the creamy sauce, tossing together until well coated. Season with kosher salt and freshly ground black pepper to taste. Cook an additional minute to heat through.
Serve the pasta hot, garnishing with diced Roma tomatoes, chopped parsley, green onions, and a sprinkle of shredded Parmesan on top. Enjoy your homemade Spicy, Creamy, Delicious: Chilis Cajun Chicken Pasta!