Preheat the oven to 350°F. Spray a 13 x 9 baking dish with cooking spray. Bring a pot of salted water to a boil.
Season the chicken with the Cajun seasoning and salt.
Heat the avocado oil in a large skillet over medium-high heat.
Cook the chicken for 7 minutes per side, or until golden brown and cooked through.
Remove from the skillet and let rest.
Add the pasta to the boiling water and cook according to package directions. Drain.
Add the butter to the skillet. Once the butter is melted, add the garlic and cook for 30 seconds. Sprinkle with the flour, and cook for 2 minutes, until the flour turns golden brown.
Add the milk, and bring to a simmer.
Reduce the heat to low, and add the cheddar cheese, American cheese, cream cheese, Cajun seasoning, salt, and pepper.
Cook, stirring frequently, until the cheese has melted. While the cheese is melting, dice the chicken.
Add the pasta back to the skillet and stir in the chicken. Transfer to the prepared baking dish.
Top with the breadcrumbs.
Bake for 30 minutes, until bubbly and lightly browned.