Cook pasta according to package directions until al dente. Drain and set aside. While pasta cooks, pat chicken dry and toss with 2 tbsp of Cajun seasoning. Heat olive oil in a large skillet over medium-high heat. Cook chicken until golden and cooked through. Remove from skillet and set aside.

In the same skillet, melt butter over medium heat. Add minced onion and cook until soft (3-4 minutes). Stir in minced garlic and cook for 1 minute more. Whisk in the flour and cook for 2 minutes, stirring constantly, to form a roux.

Slowly pour in the milk while whisking to prevent lumps, then whisk in the heavy cream. Bring to a simmer and cook until thickened. Reduce heat to low. Add the shredded cheeses one handful at a time, stirring until each addition is fully melted before adding the next. Do not boil.

Stir the remaining 1 tbsp of Cajun seasoning into the sauce. Add the cooked pasta and chicken to the skillet. Stir until everything is well-coated. Serve immediately.