Bring a large pot of salted water to a boil. Add the elbow macaroni and cook according to package instructions until al dente. Drain and set aside.
Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the diced chicken breast and sprinkle with 2 teaspoons of Cajun seasoning, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Cook, stirring occasionally, until the chicken is cooked through and lightly browned, about 6-8 minutes. Remove from heat and set aside.
In a medium saucepan, heat 2 cups of milk over medium heat until warm but not boiling. In a separate small bowl, whisk together 1/4 cup of all-purpose flour with a little cold milk to create a smooth slurry. Slowly whisk this into the warm milk, stirring constantly until the mixture thickens, about 3-5 minutes.
Reduce the heat to low and gradually stir in 2 cups shredded cheddar cheese and 1 cup shredded Monterey Jack cheese until melted and smooth. Make sure the sauce is creamy and well combined.
Add the cooked macaroni and seasoned chicken to the cheese sauce. Stir gently to combine everything evenly, making sure each piece of pasta is coated with that luscious cheese.
If you love a crunchy topping, transfer the mac and cheese mixture to a baking dish. Sprinkle 1/2 cup breadcrumbs evenly over the top. Bake in a preheated oven at 350°F (175°C) for about 15 minutes or until the top is golden and crispy.
Remove from oven and sprinkle 1/4 cup chopped green onions over the top for a fresh pop of flavor. Serve hot and enjoy the spicy, cheesy goodness.