Bring a large pot of salted water to a boil and cook the penne pasta for 2 minutes less than the package instructions for al dente. Drain the pasta and set aside.

Toss the cubed chicken breast with 1½ teaspoons of Cajun seasoning until evenly coated.

In a large skillet, melt 1 tablespoon of unsalted butter over medium heat, then add the seasoned chicken. Cook thoroughly until no longer pink inside and lightly browned. Remove from the skillet and set aside.

In the same skillet, add 1 tablespoon of olive oil and 3 tablespoons of unsalted butter. Over medium heat, add the sliced bell peppers, onion, and minced garlic. Sprinkle with 1 teaspoon of Cajun seasoning and sauté until softened and fragrant, about 5–7 minutes.

Reduce the heat to medium-low and add ½ cup of all-purpose flour to the skillet with the vegetables and butter. Stir constantly to create a roux, cooking for about 2 minutes until it becomes slightly golden but not burnt.

Gradually whisk in 2 cups of low-sodium chicken stock and ¾ cup of milk, stirring to avoid lumps. Add in the coarsely chopped roasted garlic, ½ teaspoon of salt, and ½ teaspoon of black pepper. Let the sauce simmer gently for about 5–7 minutes until it thickens.

Stir in 1 cup of grated Parmesan cheese and ¾ cup of heavy cream until melted and smooth.

Return the chicken and drained pasta to the skillet, tossing everything to coat evenly and heat through for an additional 2–3 minutes. Taste and adjust seasoning as needed.

Remove from heat, garnish with chopped green onions, and serve hot.