In a large pot of salted boiling water, cook the elbow macaroni according to the package instructions until al dente. Drain and set aside.
In a mixing bowl, combine the shredded Monterey Jack and sharp cheddar cheeses. Toss them together to create a cheese blend.
In a separate large skillet or saucepan, melt the butter (or bacon grease) over medium heat. Stir in the all-purpose flour to create a roux. Cook the roux, stirring constantly, for about 2-3 minutes or until it becomes a light golden color.
Gradually whisk in the whole milk, ensuring there are no lumps. Continue to cook and stir until the mixture thickens, which should take about 5-7 minutes.
Stir in the Cajun seasoning, adjusting the amount to your preferred level of spiciness. Keep in mind that the cheese will also add some saltiness, so you may want to taste and adjust accordingly.
Reduce the heat to low and gradually add the shredded cheese mixture to the milk mixture, stirring continuously until the cheese is fully melted and the sauce is smooth and creamy. Add the cooked elbow macaroni to the cheese sauce and gently stir until the pasta is well coated with the creamy cheese mixture.
Add into an oven safe dish, add extra shredded cheese on top and bake at 350 degrees for 25 minutes covered and 10 - 15 minutes uncovered. Enjoy!