Cook pasta in a large pot of salted boiling water until al dente. Drain and set aside.
In a large skillet over medium heat, melt the butter. Add garlic and sauté for about 1 minute until fragrant.
Add diced chicken and bell peppers to the skillet. Cook until chicken is golden and fully cooked through, and peppers are tender.
Sprinkle Cajun seasoning over the chicken and peppers, stirring to coat evenly.
Pour in milk and add cream cheese. Stir continuously until the cream cheese melts and the sauce becomes smooth.
Toss in the cooked pasta and mix until fully coated in sauce.
Stir in Parmesan cheese and cook for another minute until melted and sauce thickens slightly.
Taste and adjust seasoning with salt, pepper, or extra Cajun spice.
If using optional vegetables, fold them in and cook for 2–3 more minutes.
Serve immediately, garnished with chopped parsley or green onions if desired.